Add support for fitting estimators on dask bags

The estimators that can handle dask bags should set the bob_fit_supports_dask_bag as True. This commit also includes

• Adds a new tag: bob_fit_supports_dask_bag
• Adds a new tag: bob_checkpoint_features for when you want to always avoid checkpointing features for a specific estimator.
• Expose dask_tags, get_bob_tags in the main API
• The SampleWrapper was modified to support bob_fit_supports_dask_bag
• The CheckpointWrapper now loads estimators without losing references correctly.

bob/pipelines/__init__.py

@@ -10,15 +10,16 @@ from .sample import (
     SampleBatch,
     SampleSet,
 )
-from .wrappers import dask_tags  # noqa: F401
 from .wrappers import wrap  # noqa: F401
-from .wrappers import (
+from .wrappers import (  # noqa: F401
     BaseWrapper,
     CheckpointWrapper,
     DaskWrapper,
     DelayedSamplesCall,
     SampleWrapper,
     ToDaskBag,
+    dask_tags,
+    get_bob_tags,
 )