Added output_types in grover

[arunppsg]

Description

Summary of changes:

• added output types - torch-models requires output typy - I missed adding it earlier and I have added it now. Output types helps in distinguishing different outputs from a model, like probability values and logits.
• add optional params - vocabulary is not required for finetuning applications, hence those parameters are made os optional in GroverModel.

Type of change

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• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
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• Documentations (modification for documents)

Checklist

• My code follows the style guidelines of this project
  • Run yapf -i <modified file> and check no errors (yapf version must be 0.32.0)
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• I have performed a self-review of my own code
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• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New unit tests pass locally with my changes
• I have checked my code and corrected any misspellings

[rbharath]

LGTM

@arunppsg Feel free to merge in once ready