Add support for sharing an ORT session

[quic-suppugun]

For every instance in a model instance group a new ORT session is created. This code adds support to share a session per instance group.
This support can be enabled by defining
'share_session_between_instances' to true
in triton model config "parameters". Example:
parameters [
.....
{
key: "share_session_between_instances"
value: {string_value: "true"}
}
]

This is a global parameter and cannot be defined per instance group. The user should determine if the parameter makes sense for their setup.

When log-info option of tritonserver is set to "1", the logs will indicate that a session is mapped for the instance group during the first initialized instance and reused for other instances.
Example:
TRITONBACKEND_ModelInstanceInitialize: _0_1 (CPU device 0)
TRITONBACKEND_ModelInstanceInitialize: _0_0 (CPU device 0)
Could not find a session corresponding to instance group: _0
Created session for instance: _0_1
Mapped session for instance group: _0
Reusing session for instance: _0_0

Change-Id: I6dc509b9c2451e3dd14d45f6f150b37f50b5db89

[makerjackie]

I have compiled two images based on this PR for easy use. They are:

• docker pull jackiexiao/tritonserver:24.03-py3-onnx-share-session
• docker pull jackiexiao/tritonserver:24.03-onnx-py-cpu-onnx-share-session

The first image only replaces the ONNX backend while keeping everything else unchanged. The second image provides a smaller CPU version.

[adityagoel4512]

Hey! I was going to work on resolving the same issue with session sharing and noticed that this PR already exists, so thanks. Is there a reluctance to do this that I'm missing?

[Jacker-bro]

This is great, but it needs to be set with the enable_mem_pattern parameter to solve the performance issue.

[glebashnik]

This is really useful and it would be great to merge this.
Since this PR branch is stale I rebased this work on current main and cleaned it up a bit with some minor improvements in the new PR:
#343

It includes:

• The original session-sharing implementation
• Locking improvement for shared session initialization using RAII.
• Improved instance group name parsing without regex. This handles multi-digit instance group indexes and model
  names with regex special characters.
• Documented the new parameter.
• Added an integration test that verifies session reuse and runs inference.

I also ran local CPU/GPU benchmarks with Triton Server 2.68.0 and a ModernBERT embedder to measure memory use and throughput. Tested with enable_mem_pattern, do_copy_in_default_stream, and use_ep_level_unified_stream; these did not meaningfully affect performance for shared sessions.