Fix webgpu shared buffer issue

[apsonawane]

This pull request refactors the management of recurrent state buffers in RecurrentState to better support both graph capture (e.g., TRT-RTX) and general execution providers. The main change is to conditionally use shared or separate buffers for past/present state, improving compatibility and correctness across execution environments.

Recurrent state buffer management:

• Added a share_buffers_ flag to RecurrentState to determine whether to use the same buffer for both input and output (for graph capture) or separate buffers with swapping (for general EPs like WebGPU). [1] [2]
• Allocates and manages pasts_ and presents_ vectors conditionally based on share_buffers_, ensuring correct buffer usage and initialization.
• Refactored the Add and Update methods to handle buffer aliasing and swapping according to the execution mode, maintaining stable buffer addresses for graph capture and proper buffer updates otherwise.
• Updated the RewindTo method to zero and rebind buffers as needed, supporting both shared and separate buffer modes.

Internal state tracking:

• Introduced input_index_ and output_index_ members to track the range of recurrent state buffers within the global input/output vectors, enabling efficient buffer rebinding after rewinding or updating. [1] [2]

[Copilot]

Pull request overview

This PR fixes a WebGPU shared-buffer issue in RecurrentState by introducing a dual-mode buffer strategy: alias input/output to the same buffer when graph capture is active (TRT-RTX), or use separate past/present buffers with swap on each step for general EPs (e.g., WebGPU, which prohibits aliasing the same buffer as read-only and read-write). The implementation mirrors the established pattern in DefaultKeyValueCache.

Changes:

• Add share_buffers_ flag (derived from params_->use_graph_capture) and input_index_/output_index_ tracking to RecurrentState.
• Conditionally allocate/zero a separate pasts_ vector in non-shared mode; Add() binds input to pasts_ vs. presents_ accordingly.
• Implement swap-and-rebind in Update() and zero+rebind in RewindTo(0) for the non-shared path; shared path is unchanged.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/models/recurrent_state.h	Adds pasts_, share_buffers_, and index members for non-shared buffer tracking.
src/models/recurrent_state.cpp	Allocates separate pasts when not in graph-capture mode; swaps/rebinds buffers in Update() and RewindTo().