llvm; Refactor ExecutionMode handling

[jvesely]

Make most compiled modes private for test/development use only.
Introduce a new _LLVMPerNode execution mode.
Do not fall back to per-node mode in automatic fallback.
Use ExecutionMode helper methods rather than COMPILED mask to determine compiled mode.

[github-actions]

This PR causes the following changes to the html docs (ubuntu-latest-3.11):

diff -r docs-base/AutodiffComposition.html docs-head/AutodiffComposition.html
364c364
< <li><p>Specifying <a class="reference internal" href="LLVM.html#psyneulink.core.llvm.__init__.ExecutionMode.LLVM" title="psyneulink.core.llvm.__init__.ExecutionMode.LLVM"><code class="xref any py py-attr docutils literal notranslate"><span class="pre">ExecutionMode.LLVM</span></code></a> or <a class="reference internal" href="LLVM.html#psyneulink.core.llvm.__init__.ExecutionMode.PyTorch" title="psyneulink.core.llvm.__init__.ExecutionMode.PyTorch"><code class="xref any py py-attr docutils literal notranslate"><span class="pre">ExecutionMode.PyTorch</span></code></a> in the learn() method of a standard
---
> <li><p>Specifying <a class="reference internal" href="LLVM.html#psyneulink.core.llvm.__init__.ExecutionMode.LLVMRun" title="psyneulink.core.llvm.__init__.ExecutionMode.LLVMRun"><code class="xref any py py-attr docutils literal notranslate"><span class="pre">ExecutionMode.LLVMRun</span></code></a> or <a class="reference internal" href="LLVM.html#psyneulink.core.llvm.__init__.ExecutionMode.PyTorch" title="psyneulink.core.llvm.__init__.ExecutionMode.PyTorch"><code class="xref any py py-attr docutils literal notranslate"><span class="pre">ExecutionMode.PyTorch</span></code></a> in the learn() method of a standard
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< <p>Specifying <code class="xref any docutils literal notranslate"><span class="pre">ExecutionMode.LLVMRUn</span></code> in either the <a class="reference internal" href="Composition.html#psyneulink.core.compositions.composition.Composition.learn" title="psyneulink.core.compositions.composition.Composition.learn"><code class="xref any py py-meth docutils literal notranslate"><span class="pre">learn</span></code></a> and <a class="reference internal" href="Composition.html#psyneulink.core.compositions.composition.Composition.run" title="psyneulink.core.compositions.composition.Composition.run"><code class="xref any py py-meth docutils literal notranslate"><span class="pre">run</span></code></a>
---
> <p>Specifying <a class="reference internal" href="LLVM.html#psyneulink.core.llvm.__init__.ExecutionMode.LLVMRun" title="psyneulink.core.llvm.__init__.ExecutionMode.LLVMRun"><code class="xref any py py-attr docutils literal notranslate"><span class="pre">ExecutionMode.LLVMRun</span></code></a> in either the <a class="reference internal" href="Composition.html#psyneulink.core.compositions.composition.Composition.learn" title="psyneulink.core.compositions.composition.Composition.learn"><code class="xref any py py-meth docutils literal notranslate"><span class="pre">learn</span></code></a> and <a class="reference internal" href="Composition.html#psyneulink.core.compositions.composition.Composition.run" title="psyneulink.core.compositions.composition.Composition.run"><code class="xref any py py-meth docutils literal notranslate"><span class="pre">run</span></code></a>
diff -r docs-base/Compilation.html docs-head/Compilation.html
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< <li><p><a class="reference internal" href="LLVM.html#psyneulink.core.llvm.__init__.ExecutionMode.LLVM" title="psyneulink.core.llvm.__init__.ExecutionMode.LLVM"><code class="xref any py py-attr docutils literal notranslate"><span class="pre">ExecutionMode.LLVM</span></code></a>: Compile and execute individual nodes. The scheduling loop still runs in Python. If any of the nodes fails to compile, an error is raised. <em>NOTE:</em> Schedules that require access to node data will not work correctly.</p></li>
< <li><p><a class="reference internal" href="LLVM.html#psyneulink.core.llvm.__init__.ExecutionMode.LLVMExec" title="psyneulink.core.llvm.__init__.ExecutionMode.LLVMExec"><code class="xref any py py-attr docutils literal notranslate"><span class="pre">ExecutionMode.LLVMExec</span></code></a>: Execution of <code class="xref any docutils literal notranslate"><span class="pre">Composition.exec</span></code> is replaced by a compiled equivalent. If the <a class="reference internal" href="Composition.html"><span class="doc">Composition</span></a> fails to compile, an error is raised.</p></li>
---
> <li><p><code class="xref any docutils literal notranslate"><span class="pre">ExecutionMode._LLVMPerNode</span></code>: Compile and execute individual nodes. The scheduling loop still runs in Python. If any of the nodes fails to compile, an error is raised. <em>NOTE:</em> Schedules that require access to node data will not work correctly.</p></li>
> <li><p><a class="reference internal" href="LLVM.html#psyneulink.core.llvm.__init__.ExecutionMode._LLVMExec" title="psyneulink.core.llvm.__init__.ExecutionMode._LLVMExec"><code class="xref any py py-attr docutils literal notranslate"><span class="pre">ExecutionMode._LLVMExec</span></code></a>: Execution of <code class="xref any docutils literal notranslate"><span class="pre">Composition.exec</span></code> is replaced by a compiled equivalent. If the <a class="reference internal" href="Composition.html"><span class="doc">Composition</span></a> fails to compile, an error is raised.</p></li>
diff -r docs-base/Composition.html docs-head/Composition.html
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< <li><p><a class="reference internal" href="LLVM.html#psyneulink.core.llvm.__init__.ExecutionMode.LLVM" title="psyneulink.core.llvm.__init__.ExecutionMode.LLVM"><code class="xref any py py-attr docutils literal notranslate"><span class="pre">ExecutionMode.LLVM</span></code></a> and <a class="reference internal" href="LLVM.html#psyneulink.core.llvm.__init__.ExecutionMode.PyTorch" title="psyneulink.core.llvm.__init__.ExecutionMode.PyTorch"><code class="xref any py py-attr docutils literal notranslate"><span class="pre">ExecutionMode.PyTorch</span></code></a> can only be used in the <a class="reference internal" href="AutodiffComposition.html#psyneulink.library.compositions.autodiffcomposition.AutodiffComposition.learn" title="psyneulink.library.compositions.autodiffcomposition.AutodiffComposition.learn"><code class="xref any py py-meth docutils literal notranslate"><span class="pre">learn</span></code></a>
---
> <li><p><a class="reference internal" href="LLVM.html#psyneulink.core.llvm.__init__.ExecutionMode.LLVMRun" title="psyneulink.core.llvm.__init__.ExecutionMode.LLVMRun"><code class="xref any py py-attr docutils literal notranslate"><span class="pre">ExecutionMode.LLVMRun</span></code></a> and <a class="reference internal" href="LLVM.html#psyneulink.core.llvm.__init__.ExecutionMode.PyTorch" title="psyneulink.core.llvm.__init__.ExecutionMode.PyTorch"><code class="xref any py py-attr docutils literal notranslate"><span class="pre">ExecutionMode.PyTorch</span></code></a> can only be used in the <a class="reference internal" href="AutodiffComposition.html#psyneulink.library.compositions.autodiffcomposition.AutodiffComposition.learn" title="psyneulink.library.compositions.autodiffcomposition.AutodiffComposition.learn"><code class="xref any py py-meth docutils literal notranslate"><span class="pre">learn</span></code></a>
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< <li><p><em>True</em> – try to use the one that yields the greatesst improvement, progressively reverting to less powerful
< but more forgiving modes, in the order listed below, for each that fails;</p></li>
---
> <li><p><em>True</em> – try to use the one that yields the greatest improvement, progressively reverting to less powerful
> but more forgiving modes, trying LLVMRun, _LLVMExec, and Python.</p></li>
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< <li><p><a class="reference internal" href="LLVM.html#psyneulink.core.llvm.__init__.ExecutionMode.LLVMExec" title="psyneulink.core.llvm.__init__.ExecutionMode.LLVMExec"><code class="xref any py py-attr docutils literal notranslate"><span class="pre">ExecutionMode.LLVMExec</span></code></a> – compile and run each <a class="reference internal" href="Time.html#psyneulink.core.scheduling.time.TimeScale.TRIAL" title="psyneulink.core.scheduling.time.TimeScale.TRIAL"><code class="xref any py py-attr docutils literal notranslate"><span class="pre">TRIAL</span></code></a>, using the Python interpreter
---
> <li><p><a class="reference internal" href="LLVM.html#psyneulink.core.llvm.__init__.ExecutionMode._LLVMExec" title="psyneulink.core.llvm.__init__.ExecutionMode._LLVMExec"><code class="xref any py py-attr docutils literal notranslate"><span class="pre">ExecutionMode._LLVMExec</span></code></a> – compile and run each <a class="reference internal" href="Time.html#psyneulink.core.scheduling.time.TimeScale.TRIAL" title="psyneulink.core.scheduling.time.TimeScale.TRIAL"><code class="xref any py py-attr docutils literal notranslate"><span class="pre">TRIAL</span></code></a>, using the Python interpreter
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< equivalent the execution of the <a class="reference internal" href="#psyneulink.core.compositions.composition.Composition.execute" title="psyneulink.core.compositions.composition.Composition.execute"><code class="xref any py py-meth docutils literal notranslate"><span class="pre">execute</span></code></a> method using the Python interpreter;</p></li>
< <li><p><a class="reference internal" href="LLVM.html#psyneulink.core.llvm.__init__.ExecutionMode.LLVM" title="psyneulink.core.llvm.__init__.ExecutionMode.LLVM"><code class="xref any py py-attr docutils literal notranslate"><span class="pre">ExecutionMode.LLVM</span></code></a> – compile and run <a class="reference internal" href="#composition-nodes"><span class="std std-ref">Node</span></a> of the <a class="reference internal" href="#"><span class="doc">Composition</span></a> and their <a class="reference internal" href="Projection.html"><span class="doc">Projections</span></a>, using the Python interpreter to call the Composition’s <a class="reference internal" href="#psyneulink.core.compositions.composition.Composition.scheduler" title="psyneulink.core.compositions.composition.Composition.scheduler"><code class="xref any py py-attr docutils literal notranslate"><span class="pre">scheduler</span></code></a>,
---
> equivalent the execution of the <a class="reference internal" href="#psyneulink.core.compositions.composition.Composition.execute" title="psyneulink.core.compositions.composition.Composition.execute"><code class="xref any py py-meth docutils literal notranslate"><span class="pre">execute</span></code></a> method using the Python interpreter;
> This mode does not support Trial scope scheduling rules and should not be used outside of development or testing.</p></li>
> <li><p><code class="xref any docutils literal notranslate"><span class="pre">ExecutionMode._LLVMPerNode</span></code> – compile and run <a class="reference internal" href="#composition-nodes"><span class="std std-ref">Node</span></a> of the <a class="reference internal" href="#"><span class="doc">Composition</span></a> and their <a class="reference internal" href="Projection.html"><span class="doc">Projections</span></a>, using the Python interpreter to call the Composition’s <a class="reference internal" href="#psyneulink.core.compositions.composition.Composition.scheduler" title="psyneulink.core.compositions.composition.Composition.scheduler"><code class="xref any py py-attr docutils literal notranslate"><span class="pre">scheduler</span></code></a>,
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< <a class="reference internal" href="Condition.html"><span class="doc">Conditions</span></a> that rely on Node <a class="reference internal" href="Parameters.html"><span class="doc">Parameters</span></a> is not supported;</p></li>
---
> <a class="reference internal" href="Condition.html"><span class="doc">Conditions</span></a> that rely on Node <a class="reference internal" href="Parameters.html"><span class="doc">Parameters</span></a> are not supported;</p></li>
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< <li><p><code class="xref any docutils literal notranslate"><span class="pre">ExecutionMode.PTXrun</span></code> – compile multiple <a class="reference internal" href="Time.html#psyneulink.core.scheduling.time.TimeScale.TRIAL" title="psyneulink.core.scheduling.time.TimeScale.TRIAL"><code class="xref any py py-attr docutils literal notranslate"><span class="pre">TRIAL</span></code></a>s  for execution on GPU
---
> <li><p><a class="reference internal" href="LLVM.html#psyneulink.core.llvm.__init__.ExecutionMode.PTXRun" title="psyneulink.core.llvm.__init__.ExecutionMode.PTXRun"><code class="xref any py py-attr docutils literal notranslate"><span class="pre">ExecutionMode.PTXRun</span></code></a> – compile multiple <a class="reference internal" href="Time.html#psyneulink.core.scheduling.time.TimeScale.TRIAL" title="psyneulink.core.scheduling.time.TimeScale.TRIAL"><code class="xref any py py-attr docutils literal notranslate"><span class="pre">TRIAL</span></code></a>s  for execution on GPU
diff -r docs-base/LLVM.html docs-head/LLVM.html
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< <dt class="sig sig-object py" id="psyneulink.core.llvm.__init__.ExecutionMode.LLVMExec">
< <span class="sig-name descname"><span class="pre">LLVMExec</span></span><a class="headerlink" href="#psyneulink.core.llvm.__init__.ExecutionMode.LLVMExec" title="Permalink to this definition">¶</a></dt>
---
> <dt class="sig sig-object py" id="psyneulink.core.llvm.__init__.ExecutionMode._LLVMExec">
> <span class="sig-name descname"><span class="pre">_LLVMExec</span></span><a class="headerlink" href="#psyneulink.core.llvm.__init__.ExecutionMode._LLVMExec" title="Permalink to this definition">¶</a></dt>
diff -r docs-base/_sources/Compilation.rst.txt docs-head/_sources/Compilation.rst.txt
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<   * `ExecutionMode.LLVM`: Compile and execute individual nodes. The scheduling loop still runs in Python. If any of the nodes fails to compile, an error is raised. *NOTE:* Schedules that require access to node data will not work correctly.
<   * `ExecutionMode.LLVMExec`: Execution of `Composition.exec` is replaced by a compiled equivalent. If the `Composition` fails to compile, an error is raised.
---
>   * `ExecutionMode._LLVMPerNode`: Compile and execute individual nodes. The scheduling loop still runs in Python. If any of the nodes fails to compile, an error is raised. *NOTE:* Schedules that require access to node data will not work correctly.
>   * `ExecutionMode._LLVMExec`: Execution of `Composition.exec` is replaced by a compiled equivalent. If the `Composition` fails to compile, an error is raised.
diff -r docs-base/genindex.html docs-head/genindex.html
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>       <li><a href="LLVM.html#psyneulink.core.llvm.__init__.ExecutionMode._LLVMExec">_LLVMExec (psyneulink.core.llvm.__init__.ExecutionMode attribute)</a>
> </li>
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< </li>
<       <li><a href="LLVM.html#psyneulink.core.llvm.__init__.ExecutionMode.LLVMExec">LLVMExec (psyneulink.core.llvm.__init__.ExecutionMode attribute)</a>
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