Enable conditional optimization automatically (#15885)
### Enable conditional optimization on inputs
Label sparsity based optimization can be enabled depending on the input
inspection result.
So this PR introduce a conditional optimization path for ORTModule,
where we automatically detect data sparsity from label or embedding, and
enable the graph optimization accordingly without any user interaction.
This feature had a new requirement of delaying passing pre_grad graph
transformation config to OrtModuleGraphBuilder, from `Initialize` phase
to its `Build` phase. Because once after `_initialize_graph_builder` we
can detect the input sparsity, and make a decision to enable the
label/embed sparisty based graph optimizations.
Add UT cases for label/embed input runtime inspector.
