Add FlattenAndUnpad Op (#17845)
### Description
Add an op named `FlattenAndUnpad`.
This op implements functions:
1. Flatten the first two dims of input tensor.
2. Gather valid value from input tensor with index tensor,.
### Motivation and Context
The grad op of `PadAndUnflatten` was `GatherGrad` which is inefficient
in performance.
I implement this `FlattenAndUnpad` just to replace the `GatherGrad` as
grad of `PadAndUnflatten`.
With this op, we also can simplify the "Reshape + ShrunkenGather"
pattern to `PadAndUnflatten` in padding elimination optimizer, which
will also improve performance.
