optimize gqa cpu (microsoft#20598)

### Description
<!-- Describe your changes. -->
optimize the GQA implementation on CPU. Mainly optimization are:
1. compute attention on real total sequence length instead of maximum
sequence length in case past/present share same buffer
2. remove the mask
3. remove the transpose after attention x value

It improve the phi3 model
https://github.com/microsoft/onnxruntime-genai/blob/main/examples/python/phi3-qa.py
with max sequence length 2k/4k from 10 tps to 20 tps.

### 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. -->

onnxruntime/contrib_ops/cpu/bert/gqa_attention_base.h

@@ -105,7 +105,6 @@ class GQAAttentionBase : public AttentionBase {
     const size_t past_buff_chunk_length = static_cast<size_t>(past_buffer_sequence_length) * head_size;        // L x H
     const size_t present_buff_chunk_length = static_cast<size_t>(present_buffer_sequence_length) * head_size;  // T x H
 
-    PrepareMaskGQA(mask_data, batch_size, sequence_length, present_buffer_sequence_length, local_window_size_, seqlens_k);
     if (!past_present_share_buffer) {
       memset(present_key, 0, batch_size * kv_num_heads_ * present_buffer_sequence_length * head_size * sizeof(T));
     }
@@ -139,11 +138,6 @@ class GQAAttentionBase : public AttentionBase {
         const int output_offset = static_cast<int>(i) * sequence_length * present_buffer_sequence_length;
         T* output = attention_probs + output_offset;
 
-        // Broadcast mask data: (Bx)SxT -> (BxNx)SxT
-        memcpy(output,
-               mask_data + mask_offset,
-               probs_matrix_bytes);
-
         const T* k;
         if (packed_qkv) {
           k = K + packed_batch_stride * batch_index + kv_input_chunk_length * (head_index / kv_num_heads_factor);
@@ -216,8 +210,7 @@ class GQAAttentionBase : public AttentionBase {
     const bool is_prompt = sequence_length != 1;
     const int packed_batch_stride = packed_qkv ? (num_heads_ + 2 * kv_num_heads_) * sequence_length * head_size : 0;
     const int kv_num_heads_factor = num_heads_ / kv_num_heads_;
-    const size_t q_input_chunk_length = static_cast<size_t>(sequence_length) * head_size;                      // S x H
-    const size_t kv_input_chunk_length = static_cast<size_t>(sequence_length) * head_size;                     // L x H
+    const int kv_input_chunk_length = sequence_length * head_size;                                             // L x H
     const size_t past_buff_chunk_length = static_cast<size_t>(past_buffer_sequence_length) * head_size;        // L x H
     const size_t present_buff_chunk_length = static_cast<size_t>(present_buffer_sequence_length) * head_size;  // T x H
 
@@ -262,21 +255,17 @@ class GQAAttentionBase : public AttentionBase {
                                   i / kv_num_heads_factor);
         }
 
-        T* current_tmp_data = reinterpret_cast<T*>(tmp_buffer) + q_input_chunk_length * static_cast<int>(i);
-        const int attention_probs_offset = sequence_length * present_buffer_sequence_length * static_cast<int>(i);
-        math::MatMul<T>(sequence_length, head_size, present_buffer_sequence_length,
-                        attention_probs + attention_probs_offset,
-                        v, current_tmp_data, nullptr);
-
-        // Transpose: out(B, S, N, H_v) -> out_tmp(B, N, S, H_v)
-        T* src = current_tmp_data;
-        const int dest_offset = (batch_index * sequence_length * num_heads_ + head_index) * head_size;
-        T* dest = output + dest_offset;
-        for (int j = 0; j < sequence_length; j++) {
-          memcpy(dest, src, bytes_to_copy_trans);
-          src += head_size;
-          dest += hidden_size;
-        }
+        T* output_current = output + (batch_index * sequence_length * num_heads_ + head_index) * head_size;
+        ptrdiff_t attention_probs_offset = SafeInt<ptrdiff_t>(sequence_length) * present_buffer_sequence_length * i;
+
+        math::GemmEx<T, ThreadPool>(CblasNoTrans,
+                                    CblasNoTrans,
+                                    sequence_length, head_size, total_seqlen,
+                                    1.f, /*alpha*/
+                                    attention_probs + attention_probs_offset, present_buffer_sequence_length,
+                                    v, head_size,
+                                    0.0f /*beta*/,
+                                    output_current, hidden_size, nullptr);
       }
     });
   }