[Feature Request] Support pointer-generator networks on T5 BeamSearch

[amancini-N]

Describe the feature request

Support Pointer generator network (Get To The Point: Summarization with Pointer-Generator Networks) in T5 models

Describe scenario use case

We have note generation models for different application, and we are currently running those using a fork of onnxruntime. We would like to contribute this back on the mainline.

Subtasks

• Support pointer-generator in BeamSearch op #23123
• MultiHeadAttention op shall return attention probabilities #23124