Allow running some benchmarks without pulling in all other models

[nicolaschapados]

In particular, the run_baselines.py function doesn't run at all if ``NIXTLA_API_KEY` is not set, regardless of whether one of the Nixtla models is requested to be run. Key issue is on line 22:
from cik_benchmark.baselines.timegen import timegen1
which should be delayed until this particular model is run.