Add CogVideoX Diffusers checkpoint conversion

[anubhavsingh23906-code]

What does this PR do?

Adds a new conversion script to convert CogVideoX transformer checkpoints from Diffusers format to the CogVideoX checkpoint format.

Changes

• Added scripts/convert_diffusers_to_cogvideox.py
• Converts separate Q/K/V projection weights into combined QKV weights.
• Converts query/key LayerNorm keys to the CogVideoX format.
• Reconstructs AdaLN modulation weights and biases.
• Converts transformer layer key names to the expected CogVideoX format.
• Added CLI arguments for input and output checkpoint paths.

Validation

• Verified QKV reconstruction using synthetic tensors.
• Verified AdaLN weight rearrangement.
• Verified query/key LayerNorm key conversion.
• Verified CogVideoXTransformer3DModel can be instantiated successfully.
• Verified CLI argument parsing with --help.
• git diff --check passes.

Testing

The conversion logic has currently been validated with synthetic state dictionaries. Real checkpoint conversion has not yet been performed because a suitable source checkpoint was not available locally.

Dependencies

No new dependencies were added.

Self-review

• Reviewed the conversion logic for QKV, AdaLN, LayerNorm, and transformer key mappings.
• Confirmed the script passes basic syntax/CLI and synthetic conversion checks.
• Real checkpoint conversion remains to be validated separately.