[1/n] Reorganize sparsity module to separate weight and attention sparsity

[kaix-nv]

What does this PR do?

Type of change: ?

Overview: ?
Reorganizes the modelopt/torch/sparsity module to prepare for adding attention sparsity support. All existing weight sparsity functionality (source code and tests) is moved into a dedicated weight_sparsity subdirectory while maintaining full backward compatibility.

# These imports still work exactly as before
from modelopt.torch.sparsity import sparsify, magnitude_prune
from modelopt.torch.sparsity.mode import SparsityMode

Usage

# Add a code snippet demonstrating how to use this

Testing

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• Is this change backward compatible?: Yes/No
• Did you write any new necessary tests?: Yes/No
• Did you add or update any necessary documentation?: Yes/No
• Did you update Changelog?: Yes/No

Additional Information

[kevalmorabia97]

Apart from GitHub CICD checks, can you also verify llm_sparsity example test is working fine? pytest tests/examples/llm_sparsity

[kaix-nv]

Apart from GitHub CICD checks, can you also verify llm_sparsity example test is working fine? pytest tests/examples/llm_sparsity

I've verified that all the tests have passed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.53%. Comparing base (5adb9ba) to head (cdd34f7).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #517   +/-   ##
=======================================
  Coverage   73.52%   73.53%           
=======================================
  Files         181      182    +1     
  Lines       18207    18211    +4     
=======================================
+ Hits        13387    13391    +4     
  Misses       4820     4820           

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