[ONNX] Export module as function (#105618)
Introduce `Modularize` pass that analyzes the flat `fx.GraphModule` and creates nested
layers of sub `fx.GraphModule`s along with the `call_module` fx nodes that invokes them.
The analysis is done on the meta data "nn_module_stack", which captures the `nn.Module`
each flat `fx.Node` belongs to.
`FxOnnxInterpreter` is updated to support `call_module`. The related sub module linked
by `node.target` is exported as an ONNX model local function. The `call_module` node itself
is exported as an ONNX node, associated with the ONNX model local function by op_type.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/105618
Approved by: https://github.com/justinchuby
