Support FC (Caffe2) -> Gemm (ONNX) with variable input shape. (#16184)

Support FC (Caffe2) -> Gemm (ONNX) with variable input shape. (#16184)

Summary:
For >2D input, previously the code uses static shape captured during tracing and reshape before/after `Gemm`.
Now we add `-1` to the first `Reshape`, and uses `Shape(X) => Slice(outer) => Concat(with -1 for inner) => Reshape` for the second.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16184

Differential Revision: D14070754

Pulled By: ezyang

fbshipit-source-id: 86c69e9b254945b3406c07e122e57a00dfeba3df