[travis] Record code coverage and display on README

[willprice]

This PR adds code coverage to the travis builds and adds an accompanying badge.

A few details of why I've implemented it the way I have:

To test against the installed version of the package (which is
preferably since this can catch installation file inclusion bugs) we
have to deal with quirks of coverage.py. By default the ecosystem is
set up to do development coverage tests (e.g. in src, or by using pip install -e . or python setup.py develop).

We want to test against the version installed, to do this we have to
find the install path which we'll then pass to the --cov arg added by
pytest-cov. To do this, we have to cd out of the current folder, and
import the installed version and get it's install path (if we don't cd
out, then we end up getting the existing directory in the cwd since by
default cwd is on sys.path)

Once we have the install path, we pass that to the --cov of pytest,
however we also want to rewrite the paths for codecov to pick them up on
the website, if they don't have to code coverage with local paths, they
don't register is properly.

In order to that, we have a .coveragerc file that has the contents:

[paths]
source =
    torchvision
    /**/site-packages/torchvision

This tells codecov to treat all paths with those prefixes as the same,
so anything like
/home/travis/miniconda/envs/test-environment/lib/python2.7/site-packages/torchvision/models/__init__.py
would be treated the same as
torchvision/models/__init__.py after running the coverage combination
command. The first path seems to be special, in that all subsequent
paths are rewritten to that one.

Once we collect coverage, we then run coverage run to rewrite the
installation paths in the coverage file, .coverage, to those expected
by codecov.

Phew. And that's it.

N.B: Whilst adding test/init.py does solve the issue, it also results in
PWD being added to the path, and then we'll test the development version
of the package, and not the installed version (see
https://docs.pytest.org/en/latest/goodpractices.html#tests-as-part-of-application-code)

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@84896a6). Click here to learn what that means.
The diff coverage is n/a.

@@           Coverage Diff            @@
##             master    #703   +/-   ##
========================================
  Coverage          ?   43.1%           
========================================
  Files             ?      29           
  Lines             ?    2909           
  Branches          ?     476           
========================================
  Hits              ?    1254           
  Misses            ?    1581           
  Partials          ?      74

---

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[willprice]

If/When this PR is merged in, someone will need to add a new config entry to travis.

1. Visit https://codecov.io/gh/pytorch/vision
2. Login and add vision as a repo (you can go directly to https://codecov.io/gh/pytorch/vision)
3. Copy value of the CODECOV_TOKEN="xxxxxxxx-a3df-45fb-xxxx-114d63b284af" token
4. Open up https://travis-ci.com/pytorch/vision and go to the settings page and add the token value with the name CODECOV_TOKEN, make sure the option for this to show up in builds is not ticked (this is a secret key)

Et voila, it should all be hunky dory.

[fmassa]

This looks good to me, thanks!

I'll let @soumith comment on this as well, as it involves a few changes that I'm not very familiar with.

[willprice]

Looks like the travis python 2.7 build timed out and needs to be rerun.

[fmassa]

(Closing and reopening the PR also rerun the tests)

[willprice]

@fmassa, thanks, didn't think of that!

[soumith]

thanks @willprice !

[willprice]

@soumith Thanks for merging :)