[PROTON] Build proton by default

[Jokeren]

Summary of changes:

1. Remove cuda.h from nvidia/backend/include.
2. Download three packages: cudacrt, cudart, and cupti from nvidia's conda channel.
3. Remove TRITON_BUILD_PROTON to build proton by default.

[fkouteib]

@Jokeren This change caused nightly wheels failure on Python3.7 wheel build.

File "/tmp/pip-build-env-of9ihb6h/overlay/lib/python3.7/site-packages/setuptools/build_meta.py", line 338, in run_setup
exec(code, locals())
File "", line 446, in
File "", line 258, in download_and_copy
TypeError: copytree() got an unexpected keyword argument 'dirs_exist_ok'
[end of output]

'dirs_exist_ok' in 'shutils.copytree()' was added in Python 3.8. Nightly wheel builds every Python version from 3.7 to 3.12.

@ptillet Does it make sense to continue building Triton nightly wheels for Python 3.7 going forward? It's been EOL'ed since 6/27/2023. PyTorch stable release only goes back to Python 3.8. Ubuntu 20 ships with Python 3.8 and Ubuntu 22 ships with Python 3.10.

[jlebar]

Phil has told me we want to support the oldest supported Python, which seems like not 3.7. I think we can delete it.

[fkouteib]

Thanks Justin. PR #3778 to fix it