Release 0.9.1

Bug fixes and other changes

Depends on apache-beam[gcp]>=2.6,<3.
Requires pre-installed TensorFlow >=1.10,<2.
Updated ExampleCount to use the batch dimension as the example count. It
also now tries a few fallbacks if none of the standard keys are found in the
predictions dictionary: the first key in sorted order in the predictions
dictionary, or failing that, the first key in sorted order in the labels
dictionary, or failing that, it defaults to zero.
Fix bug where we were mutating an element in a DoFn - this is prohibited in
the Beam model and can cause subtle bugs.
Fix bug where we were creating a separate Shared handle for each stage in
Evaluate, resulting in no sharing of the model across stages.