Fixes for partitioner with symbolic shapes (#86425)
- supports saving symint (and symfloat..) values between fw/bwd, using sketchy logic that probably needs to be improved but seems to work so far
- sets a correct weight=1 for sym nodes for cost purposes
- lets user functions return symints/floats (but if the same symfloat is saved for backward, that gets duplicated annoyingly)
- makes partitioning decisions based on observed trace-time sizes without guarding! (this is sketchy, but it isn't clear that it will lead to bad partitioning choices either)
- improves infra for tracking symint-family of types: is_sym_node() and _py_sym_types
Pull Request resolved: https://github.com/pytorch/pytorch/pull/86425
Approved by: https://github.com/ezyang
