Add AdaBelief optimizer

[harsh-sutariya]

• Implement AdaBelief optimizer following the NeurIPS 2020 paper
• Add unit tests following MLX standard patterns
• Update optimizer documentation to include AdaBelief

Fixes #2479

Proposed changes

• Added AdaBelief class to python/mlx/optimizers/optimizers.py
• Implemented with bias correction enabled by default (matching paper recommendations)
• Supports decoupled weight decay
• Added unit tests in python/tests/test_optimizers.py
• Updated documentation in docs/src/python/optimizers/common_optimizers.rst

Checklist

• I have read the CONTRIBUTING document
• I have run pre-commit run --all-files to format my code / installed pre-commit prior to committing changes
• I have added tests that prove my fix is effective or that my feature works
• I have updated the necessary documentation (if needed)