[WebNN EP] Support MultiHeadAttention(MHA) (#24079)
### Description
<!-- Describe your changes. -->
Adds support for MultiHeadAttention via WebNN matmul, transpose,
reshape, and other operations that follow the logic in the MHA subgraph
below
```
Abbreviatios: B is batch_size, S is sequence_length, W is hidden_size, P is past_sequence_length
N is number of attention heads, H is head size, and W=N*H, h=Sqrt(H)
Notes: If the datatype of the inputs (qkv and past kv) is float16, we cast them to float32 to ensure data precision.
query key value
| | |
q_Reshape k_Reshape v_Reshape (shape=B,S,H,N)
| | |
q_Transpose k_Transpose v_Transpose (perm=0,2,1,3)
\ / |
\ / |
present_key<---\----Concat <---------|----past_key
| | |
| opt_k_transpose |
\ (0,1,3,2) |
\ / | past_value
qk_MatMul | /
| scale | /
| / | /
qk_Div Concat------> present_value
| |
| /
Add <----------/---------------attention_bias
| /
Softmax /
\ /
\ /
qkv_MatMul
|
Transpose (perm=0,2,1,3)
|
Reshape---(shape=B,P,W)
|
output
```
### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->
