Remove dependency on tqdm

[fmassa]

Fixes #766

[alsrgv]

@fmassa, it looks like if tqdm is not installed, this actually does not work because tqdm fallback does not have no-argument constructor.

I found out because it broke our integration tests.

[soumith]

🤕 Francisco can you please land another fix and please test it this time. Going live on binaries with broken things like this is not a good thing.

[XavierAtShanghai]

I see the following error:

File ".../local/lib/python2.7/site-packages/torchvision/datasets/utils.py", line 73, in download_url
    reporthook=gen_bar_updater(tqdm())
TypeError: __init__() takes at least 2 arguments (1 given)

[fmassa]

I'm fixing this, sorry about the breakage.
My testings were indeed not enough, I'm sorry about that

[fmassa]

@alsrgv this should fix it #770

[alsrgv]

@fmassa, it works now - thanks!