[docs] Add auto API generator

[Routhleck]

This pull request includes significant updates to the documentation and the addition of docstrings to the pinnx.metrics module. The most important changes include the addition of new reStructuredText (reST) templates and modules, updates to the documentation configuration, and enhancements to the pinnx.metrics functions.

Documentation Enhancements:

• docs/_templates/classtemplate.rst: Added a new template for class documentation with hidden sections and module references.
• docs/apis/pinnx.callbacks.rst, docs/apis/pinnx.fnspace.rst, docs/apis/pinnx.grad.rst, docs/apis/pinnx.metrics.rst, docs/apis/pinnx.rst: Added new reST files for various pinnx modules with autosummary directives for generating documentation. [1] [2] [3] [4] [5]
• docs/auto_generater.py: Added a new Python script to automatically generate documentation for modules, including functions to write module summaries and subsections.
• docs/conf.py: Updated the Sphinx configuration to include the newly created auto_generater script and ensure the apis/ directory is created.
• docs/index.rst: Added a toctree directive to include API documentation for various pinnx modules.

Code Enhancements:

• pinnx/metrics.py: Added docstrings to various metric functions to describe their purpose and usage. [1] [2] [3] [4] [5]