Add Continuous Decoding support in GQA (#21523)
### Description
This PR will add support for Continuous Decoding for batch_size = 1
input. From now on, GQA can take arbitrary length input using seqlens_k
as total_sequence_length - 1 and the sequence length of qkv as
new_sequence_length.
**This change will not affect the default behavior of GQA**
### Motivation and Context
Prior to this change it was impossible to support sequence_length > 1
inputs when past context was given. This use case is essential to making
continuous decoding work, which is one of our current efforts in
ORT-GenAI.
