Fix makedirs() condition in contrib.

[cbalint13]

This fix contrib.download when only the filename is specified as destination.

• See the failure and debug below:

------------->>>>> tvm/contrib/download.py <<<<--------------
    dirpath = os.path.dirname(path)
    print ("DBG: dirpath:[%s] path:[%s]" % (dirpath,path))
    if not os.path.isdir(dirpath):
        print ("DBG: not os.path.isdir(dirpath) is [True]")
        os.makedirs(dirpath)
-------------<<<<< tvm/contrib/download.py >>>>--------------

[cbalint@tune]$ ./test_tune.py 
Downloading from url https://sample.com/nn/my.weights to my.weights
DBG: dirpath:[] path:[my.weights]
DBG: not os.path.isdir(dirpath) is [True]

Traceback (most recent call last):
  File "./test_tune.py", line 137, in <module>
    download(WEIGHTS_URL, WEIGHTS_NAME)
  File "/usr/lib64/python3.7/site-packages/tvm/contrib/download.py", line 65, in download
    os.makedirs(dirpath)
  File "/usr/lib64/python3.7/os.py", line 221, in makedirs
    mkdir(name, mode)
FileNotFoundError: [Errno 2] No such file or directory: ''

[icemelon]

You can use not dirpath to check whether a string is empty.
Could you also put the non-empty check before the os.path.isdir check?

[cbalint13]

Changed as desired. There are other ways to check if empty too.

[icemelon]

Thanks @cbalint13