[Doc] Various improvements to Tune Pytorch CIFAR tutorial (ray-projec…

…t#50316)

## Why are these changes needed?

* added missing pip requirements
* surpressed verbose outputs with a few `hide-cell` tags
* improved overall narrative flow
* factored out dataset creation code to make things tidy
* added net.train() net.eval() toggles
* added weight decay, as a treat
* consistent use of nn.DataParallel
* deduplicated discussion about search spaces
* fixed the smoke tests

## Related issue number

N/A
## Checks

- [X] I've signed off every commit(by using the -s flag, i.e., `git
commit -s`) in this PR.
- [ ] 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 added any new APIs to the API Reference. For example, if I
added a
method in Tune, I've added it in `doc/source/tune/api/` under the
           corresponding `.rst` file.
- [ ] 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 :(

Signed-off-by: Ricardo Decal <rdecal@anyscale.com>
Signed-off-by: Ricardo Decal <crypdick@users.noreply.github.com>