fix bug when CUDA_HOME is None

[xiawu]

when install apex on cpu platform(no support CUDA), the installation throw the following error:

Running command python setup.py egg_info
Traceback (most recent call last):
File "", line 2, in
File "", line 34, in
File "/home/x/x/apex/setup.py", line 131, in
_, bare_metal_version = get_cuda_bare_metal_version(CUDA_HOME)
File "/home/x/x/apex/setup.py", line 17, in get_cuda_bare_metal_version
raw_output = subprocess.check_output([cuda_dir + "/bin/nvcc", "-V"], universal_newlines=True)
TypeError: unsupported operand type(s) for +: 'NoneType' and 'str'

This PR will fix it.

[crcrpar]

@xiawu out of curiosity, which features would you use after CPU-only installation?

[xiawu]

@xiawu out of curiosity, which features would you use after CPU-only installation?

running some projects that is dependent on apex, such as stable-diffusion. Because my GPU is old and has not enough memory.

[OneQinG12]

Error reporting：
torch.version = 1.5.0+cu101
usage: setup.py [global_opts] cmd1 [cmd1_opts] [cmd2 [cmd2_opts] ...]
or: setup.py --help [cmd1 cmd2 ...]
or: setup.py --help-commands
or: setup.py cmd --help

error: no commands supplied