CelebA: track attr names, support split="all", code cleanup

[djsutherland]

Some minor improvements to the CelebA class:

• Track the names of the different attributes in self.attr_names.
• Allow passing split="all" to load all of train/val/test at once.
• Be a little less repetitive in the constructor, and (IMO) clearer by not setting self.attr/etc first to a dataframe and then overriding them with a tensor....

[codecov-io]

Codecov Report

Merging #1008 into master will increase coverage by 0.09%.
The diff coverage is 6.25%.

@@            Coverage Diff             @@
##           master    #1008      +/-   ##
==========================================
+ Coverage   62.63%   62.73%   +0.09%     
==========================================
  Files          65       65              
  Lines        5101     5101              
  Branches      765      766       +1     
==========================================
+ Hits         3195     3200       +5     
+ Misses       1683     1679       -4     
+ Partials      223      222       -1

Impacted Files	Coverage Δ
torchvision/datasets/celeba.py	17.24% <6.25%> (+1.14%)	⬆️
torchvision/transforms/functional.py	71.12% <0%> (+1.21%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 8a64dbc...d7a703b. Read the comment docs.

[fmassa]

LGTM, thanks a lot!