Minor code cleanup and refactoring

[Bebra777228]

Code Cleanup and Refactoring

What was done:

• Removed unused imports
• Removed or merged duplicate tests
• Conducted minor refactoring

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Details:

• The tests test_cli_Vocals_output_name_argument and test_cli_Instrumental_output_name_argument were removed. This decision was made because the code already includes two tests for renaming all stems for Demucs and other architectures. Therefore, a separate test for individual stems was deemed redundant.
• The test test_cli_use_autocast_argument was also removed, as an identical test already exists in the code.
• The tests test_inverse_device_handling and test_inverse_output_shape were combined into a single function, test_inverse_stft, since they were performing the same action.