[Train] Allow train methods to be called outside of the session

[amogkam]

Updates to address @worldveil's feedback:

• Include import train.torch in the docs
• Allow methods in session.py to be called outside of the session with sensible defaults. These will no longer raise an error.

Why are these changes needed?

Related issue number

Checks

• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[bveeramani]

‼️ ACTION REQUIRED ‼️

We've switched our code formatter from YAPF to Black (see #21311).

To prevent issues with merging your code, here's what you'll need to do:

1. Install Black

pip install -I black==21.12b0

2. Format changed files with Black

curl -o format-changed.sh https://gist.githubusercontent.com/bveeramani/42ef0e9e387b755a8a735b084af976f2/raw/7631276790765d555c423b8db2b679fd957b984a/format-changed.sh
chmod +x ./format-changed.sh
./format-changed.sh
rm format-changed.sh

3. Commit your changes.

git add --all
git commit -m "Format Python code with Black"

4. Merge master into your branch.

git pull upstream master

5. Resolve merge conflicts (if necessary).

After running these steps, you'll have the updated format.sh.

[worldveil]

Just checking, this will allow it to be created inside Jupyter notebook, outside of a train run, correct?

[amogkam]

@worldveil yep thats right.

[worldveil]

Anything I can do to unblock here?

[amogkam]

Comments are addressed, please take another look!

[matthewdeng]

Take a look at failing tests?

[matthewdeng]

Nice, LGTM pending successful tests.

[amogkam]

Failing tests are unrelated, going to merge.