[doc] Add missing docstrings to zero3 chunk search utilities

[yhna940]

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🚨 Issue number

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📝 What does this PR do?

This PR adds missing docstrings to the colossalai/zero/gemini/chunk/search_utils.py file. The previous version lacked proper documentation for some functions, making it difficult for users to understand their purpose and usage.

The changes made in this PR include:

• Added detailed docstrings to _filter_exlarge_params, _get_unused_byte, _tensor_numel, and search_chunk_configuration functions.
• The docstrings now provide information about the function's purpose, input arguments, and return values.
• The added documentation will help users understand the implementation and usage of these functions better.

Please review and let me know if any modifications are required.

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• I have performed a self-review of my code
• I have added thorough tests.
• I have added docstrings for all the functions/methods I implemented

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