Fixed ROI Pooling Output Shape to Consider Multiple ROIs (#2350)

[VarunS1997]

What does this PR do?

Keras ROI Pooling was not including multiple ROIs in the output. Instead, only the first ROI was included. Documentation for underlying function that created per-feature-map ROI Pools was similarly incorrect. Thus, I also corrected the documentation for _pool_single_sample to match the expectations provided in ROIPooler.call.

Corresponding test cases were updated to reflect the update and a new test was added to test this specific situation.

Fixes #2350.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline, Pull Request section?
• Was this discussed/approved via a Github issue? Please add a link to it if that's the case. (Possible indentation error #2350)
• Did you write any new necessary tests?
• If this adds a new model, can you run a few training steps on TPU in Colab to ensure that no XLA incompatible OP are used?

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[divyashreepathihalli]

LGTM! thanks.