Summary:
~~This is work in progress due to its dependency on multiple pending PRs.~~

- [x] ONNX: Relax constraint on subgraph input/output type & shape check. onnx/onnx#2009
- [x] PyTorch: Add infra to test_pytorch_onnx_caffe2.py to test ScriptModule models. #20256

This PR should partially resolve #17531. However, ideally we shouldn't need to put cast(and reshape) node to help the conversion for loop condition.

- Added cast node for condition values before entering loop node. The ONNX spec only accepts Bool type, while in PyTorch if the condition value is an output from other node it could potentially have any integral type.
- Tidying up the exported ONNX loop subgraph input type & shape. According to ONNX spec, input "M" is exported as 0-d scalar tensor with type int64. input "Cond" is exported as incomplete tensor of type Bool without shape information. This is because through out the iteration, the rank of condition value is dynamic, either 0-d or 1-d, as long as it holds a single value.
Pull Request resolved: #20445

Differential Revision: D15534188

Pulled By: houseroad

fbshipit-source-id: d174e778529def05ee666afeee4b8fb27786e320

Summary:
- [x] Add tests after #20256 is merged

- Support exporting ScriptModule with inputs/outputs of arbitrarily constructed tuples.

- Moved the assigning of output shapes to after graph conversion to ONNX is completed. By then all tuples in the IR has already been lowered by the pass ```_jit_pass_lower_all_tuples```. If assigning output shapes is required to happen before that, we'll need to hand parse the tuple structures in the graph, and repeat the same logic in ```_jit_pass_lower_all_tuples```. Handling inputs is easier because all tuple information is encoded within the input tensor type.

- Swap the order of ```_jit_pass_lower_all_tuples``` and ```_jit_pass_erase_number_types```. Ops like ```prim::TupleIndex``` relies on index being a scalar. ```_jit_pass_erase_number_types``` will convert these kind of scalars to tensors.
Pull Request resolved: #20784

Reviewed By: zrphercule

Differential Revision: D15484171

Pulled By: houseroad

fbshipit-source-id: 4767a84038244c929f5662758047af6cb92228d3