Optimize SCE loss compute (#15401)
### Optimize SCE loss compute
Compute optimization based on label data sparsity:
- Insert ShrunkenGather before SCELoss node, to filter out invalid
labels for compute.
- Support ShrunkenGather upstream.
- Added test for the above.
- Added flag to enable label sparsity optimization with env var, by
default disabled now. Will enable after comprehensive benchmarking
later.
- Extract common logic into test_optimizer_utils.h/cc from
core/optimizer/compute_optimzier_test.cc, then the common functions can
be shared by both core/optimizer/compute_optimzier_test.cc and
orttraining/core/optimizer/compute_optimzier_test.cc
- Extract common logic into shared_utils.h/cc: `GetONNXOpSetVersion` and
`Create1DInitializerFromVector`
### Motivation and Context
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