Support `auto_lr_find` with `num_workers` > 0

[ejm714]

auto_lr_find does not support DDP. We set the multiprocessing context based on the number of workers. This leads to a scenario where a user cannot use auto_lr_find with multiple workers, even though this would be possible if the multiprocessing context were set to None.

A couple options:

• infer the multiprocessing context based on number of GPUs instead of number of workers
• allow the user to explicitly set the multiprocessing context
• do not support multiprocessing until PTL supports this for auto lr find