Resolve `'audioldm2' is a package and cannot be directly executed` by adding __main__.py

[lingchuL]

For now when using audioldm2 in command line directly, an error will be raised: python.exe: No module named audioldm2.__main__; 'audioldm2' is a package and cannot be directly executed This error is also been mentioned at this issue. By adding the __main__.py , the error would be solved and make audioldm2 easier to run in command line~