@Juans Are you trying to use multiprocessing to parallelize processing in the forward pass on a single node, or are you trying to do this in a distributed setting?
Is there any more info you can provide about the function you are trying to parallelize in the forward pass? A search indicated that this error is thrown when you attempt to pickle the pool object itself (say the function you are trying to parallelize results in pickling the pool).
I am looking for some examples of this behavior, but the recommended method would be to parallelize the entire training iteration (the entire forward pass and backward pass on a single batch) using DDP instead of just parallelizing one part of the fwd pass.