Specify package version

[AnirudhDagar]

Issue #, if available:

>>> import autogluon_assistant
>>> print(autogluon_assistant.__version__)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: module 'autogluon_assistant' has no attribute '__version__'

Description of changes:
This PR reads the package version specified in the pyproject.toml file during installation and sets the __version__ attribute accordingly.

I also made some other minor fixes, removing unused import etc.

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