Various small improvements

[alexhernandezgarcia]

Just two small updates:

• Allow the proxy format to be a list too (not only a tensor)
• Small fix in the common tests to avoid issues with non-copied states.

Note: these changes are applied on top of PR https://github.com/alexhernandezgarcia/gflownet/pull/283 (to include the updated black formatting)

Tests, black and isort

• [x] python -m pytest ./tests/ (locally)
• [x] black ./gflownet/
• [x] black ./tests/
• [x] isort --profile black ./gflownet/
• [x] isort --profile black ./tests/

[alexhernandezgarcia]

The changes in this PR are already included (or further updated) in PR https://github.com/alexhernandezgarcia/gflownet/pull/285. Closing this one.