fix: import error when missing tensorflow (now optional)

dmlcloud/core/callbacks.py

@@ -9,7 +9,6 @@
 import torch
 from omegaconf import OmegaConf
 from progress_table import ProgressTable
-from torch.utils.tensorboard import SummaryWriter
 
 from ..git import git_diff
 from ..util.logging import DevNullIO, experiment_header, general_diagnostics, IORedirector
@@ -426,8 +425,14 @@ class TensorboardCallback(Callback):
 
     def __init__(self, log_dir: Union[str, Path]):
         self.log_dir = Path(log_dir)
+        try:
+            from torch.utils.tensorboard import SummaryWriter  # noqa: F401
+        except ImportError:
+            raise ImportError('tensorflow is required for the TensorboardCallback')
 
     def pre_run(self, pipe):
+        from torch.utils.tensorboard import SummaryWriter
+
         self.writer = SummaryWriter(log_dir=self.log_dir)
 
     def post_epoch(self, stage: 'Stage'):