Wandb step handling bugfix and feature

[sjmielke]

This PR contains:

1. a bugfix to the parsing of args that allows for the passing of the resume_from argument whose value may contain =, which under the current implementation gets oversplit resulting in an error.
2. For projects without rewinding, where the above is not an option, allow for the explicit passing of the step during wandb.run.log instead, defaulting to None, but if set through the same wandb_args list allowing the editing of reports (to not have to add more logic around that, I think the behavior of having the report be based on the latest run is quite reasonable anyway) and passing the split-off value that way.

Please let me know what you think about these changes, if edits are needed, or if the process works somewhat differently here :)

[CLAassistant]

All committers have signed the CLA.

[baberabb]

Thanks for this! LGTM, assuming you have tested it.

[sjmielke]

Both changes tested as I was developing them, working as expected.