Introduce shrunken gather operator (#15396)
### Introduce shrunken gather operator
Exist Gather operator schema won't guarantee output element count will
be smaller than input element count.
Actually, it is possible output element count >, =, or < input element
count.
For some cases we know for sure output element count MUST be <= input
element count, we will upstream those Gather operators to reduce compute
flops.
So this PR introduces an ShrunkenGather which explicitly guarantee
output count will be smaller than input count. The operator add
additional restriction on inputs, but still re-use existing Gather's
implementations plus input check during runtime.
This is a requirement for subsequent optimization (Draft PR:
https://github.com/microsoft/onnxruntime/pull/15401) we will do for
label sparsity and embedding sparsity.
