Make dm_control an extra dependency

[krzentner]

Because dm_control needs to be installed from vcs or tarball, garage
cannot be directly installed in pipenv (see this
issue).

I've copied the example that used our dm_control wrapper, modified
the original example to use gym, and modified the copy to clarify the
extra dependency.

I've also modified the garage.env.dm_control module to report a
helpful error message if dm_control isn't found.

pip install garage[all] will still install dm_control.

[codecov]

Codecov Report

Merging #828 into master will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #828      +/-   ##
==========================================
- Coverage   73.48%   73.44%   -0.04%     
==========================================
  Files         178      178              
  Lines       10295    10295              
  Branches     1304     1304              
==========================================
- Hits         7565     7561       -4     
- Misses       2365     2368       +3     
- Partials      365      366       +1

Impacted Files	Coverage Δ
src/garage/misc/special.py	30.3% <0%> (-3.04%)	⬇️
src/garage/tf/optimizers/first_order_optimizer.py	65.27% <0%> (-2.78%)	⬇️

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[ryanjulian]

It can be installed with pipenv, but requires the --pre flag. pipenv refuses to match dependencies <1.0 by default.

If this is the only package which requires the flag, then I'm happy to move it into an optional. If the mainline packages require --pre anyway, then I want to avoid running in circles around the issue.

[zhanpenghe]

This looks great!

[krzentner]

Ah, I see. This is indeed the only package which requires --pre. As far as I can tell, versions before 1.0 are still considered release versions, but versions like e.g. 2.0rc1 aren't.

[gitanshu]

I don't need to use --pre for anything with pipenv if I install pipenv from master. OTOH, if I use the last release of pipenv (2018.11.26), even --pre doesn't work for dm_control.

[krzentner]

I don't need to use --pre for anything with pipenv if I install pipenv from master. OTOH, if I use the last release of pipenv (2018.11.26), even --pre doesn't work for dm_control.

Ah, thanks for testing this. I just know that I couldn't get it to work with the latest release. If a pipenv release fixes this soon, then I'd be happy to drop this change.

[ryanjulian]

I think the best move is probably to advise people to use pre-release pipenv. I don't see anything on their GitHub indicating a release is imminent, but working around this is kind of a PITA for a normal user.

ref: pypa/pipenv#2596

[gitanshu]

I opened a PR for that #839

[krzentner]

After experimenting with pre-release pipenv for a bit, I don't think advising people to use it is a good idea. It (and prerelease pip) break sometimes, and are incredibly difficult to fix afterwards.

[ryanjulian]

Okay, let's roll forward with this fix then.

[krzentner]

Okay, I'll merge this then. It's worth mentioning that all of garage's functionality can still be used in pipenv by directly installing the required dm_control version. This works since pipenv (at least since version 2018.11) supports installing packages from url, it just doesn't support dependencies of those packages being url / version control.