Fix Numerical issues in MaskPPO Training

[Sarthak-Dayal]

Description

High-level description: Improve numerical stability by mirroring the logic used by TorchRL, which uses -inf instead of a small number and handles masking better.

Details of changes:

1. Use -inf instead of -1e6 as logits when we want to mask
2. In __init__, store the original "pristine" logits, then recompute masked logits each time using the original pristine copy in apply_masking
3. Remove manual self.probs fiddling since this is handled internally
4. Re-write entropy to ensure that there are no weird NaN issues, mirror logic from torchRL that clamps invalid logits with min_real (the minimum possible value of the dtype of the logits), re-normalizes, and then computes entropy only over valid entries.

Context

closes #81

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Types of changes

• 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 change)
• Documentation (update in the documentation)

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