Fix subgraph index issue in TRT (#14305)
Subgraph index in TRT engine name keeps increasing when multiple
sessions are created for the same model, which causes TRT engine not
being reused and new engine is created again. The issue is because
trt_model_id_generator_ is defined globally.
This PR made following changes and improvements,
1. Define subgraph index as local variable thus it won't be shared
across sessions.
2. Decouple subgraph index from hash id generator
3. Call hash id generator once at the beginning of GetCapability since
hash id is shared between TRT subgraphs and there is no need to call it
for each subgraph
fix https://github.com/microsoft/onnxruntime/issues/14269
