Enhance Makefile with 'format_diff' Option and Improved Readability (#7394)

### Description:

This PR introduces a new option format_diff to the existing Makefile.
This option allows us to apply the formatting tools (Black and isort)
only to the changed Python and ipynb files since the last commit. This
will make our development process more efficient as we only format the
codes that we modify. Along with this change, comments were added to
make the Makefile more understandable and maintainable.

### Issue:

N/A

### Dependencies:

Add dependency to black.

### Tag maintainer:

@baskaryan

### Twitter handle:

[kzk_maeda](https://twitter.com/kzk_maeda)

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Co-authored-by: Bagatur <[email protected]>