ConcreteInterpreterSessionImpl::pickle is called for Dicts containing Tensors

[d4l3k]

ConcreteInterpreterSessionImpl::pickle seems to be quite expensive in our stack: https://fburl.com/strobelight/bfwdctiy maybe there is some opportunities for optimizations

We call pickle for Dicts containing torch tensors. This is a fairly expensive operation so we should have some way of handling it in a more performant way.