[ONNX]Fix export of flatten (#40418)
Summary:
Shape is passed to _reshape_to_tensor as a Constant and cannot infer shape of the input when model is exported with dynamic axes set. Instead of a Constant pass output of a subgraph Shape-Slice-Concat to compute the shape for the Reshape node in _reshape_to_tensor function.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/40418
Reviewed By: hl475
Differential Revision: D22480127
Pulled By: houseroad
fbshipit-source-id: 11853adb6e6914936871db1476916699141de435
