Update setup.py

[tejhande]

Variable Naming: Renamed RUN_BUILD to BUILD_MONAI for clarity. Extension Module Handling: Moved the logic for defining extension modules (ext_modules) inside the conditional block, ensuring that they are only defined when BUILD_MONAI is enabled. ImportError Handling: Added handling for ImportError when importing Torch, providing a warning message if Torch is not available. Package Data Inclusion: Included all files in monai/csrc directory in the package_data section to ensure that all relevant files are included in the package distribution. These changes enhance the clarity, correctness, and robustness of the setup script, ensuring that it functions correctly and efficiently for building the MONAI framework.

Fixes # .

Description

A few sentences describing the changes proposed in this pull request.

Types of changes

• [x] Non-breaking change (fix or new feature that would not break existing functionality).
• [ ] Breaking change (fix or new feature that would cause existing functionality to change).
• [ ] New tests added to cover the changes.
• [ ] Integration tests passed locally by running ./runtests.sh -f -u --net --coverage.
• [ ] Quick tests passed locally by running ./runtests.sh --quick --unittests --disttests.
• [ ] In-line docstrings updated.
• [ ] Documentation updated, tested make html command in the docs/ folder.