Fix XFormers attention for Llama-2 70B (GQA)

Updated XFormers MonkeyPatch to handle GQA as used in Llama-2 70B. All the updated code is taken directly from the Transformers library: 07360b6c9c (diff-06392bad3b9e97be9ade60d4ac46f73b6809388f4d507c2ba1384ab872711c51) from their llama_modeling.py file.

src/axolotl/monkeypatch/llama_attn_hijack_xformers.py

@@ -9,6 +9,7 @@ from typing import Optional, Tuple
 import torch
 import transformers.models.llama.modeling_llama
 from torch import nn
+import torch.nn.functional as F
 
 try:
     import xformers.ops
@@ -38,19 +39,39 @@ def xformers_forward(
     # pylint: disable=duplicate-code
     bsz, q_len, _ = hidden_states.size()
 
+    if self.pretraining_tp > 1:
+        key_value_slicing = (self.num_key_value_heads * self.head_dim) // self.pretraining_tp
+        query_slices = self.q_proj.weight.split((self.num_heads * self.head_dim) // self.pretraining_tp, dim=0)
+        key_slices = self.k_proj.weight.split(key_value_slicing, dim=0)
+        value_slices = self.v_proj.weight.split(key_value_slicing, dim=0)
+
+        query_states = [F.linear(hidden_states, query_slices[i]) for i in range(self.pretraining_tp)]
+        query_states = torch.cat(query_states, dim=-1)
+
+        key_states = [F.linear(hidden_states, key_slices[i]) for i in range(self.pretraining_tp)]
+        key_states = torch.cat(key_states, dim=-1)
+
+        value_states = [F.linear(hidden_states, value_slices[i]) for i in range(self.pretraining_tp)]
+        value_states = torch.cat(value_states, dim=-1)
+
+    else:
+        query_states = self.q_proj(hidden_states)
+        key_states = self.k_proj(hidden_states)
+        value_states = self.v_proj(hidden_states)    
+
     query_states = (
-        self.q_proj(hidden_states)
+        query_states
         .view(bsz, q_len, self.num_heads, self.head_dim)
         .transpose(1, 2)
     )
     key_states = (
-        self.k_proj(hidden_states)
+        key_states
-        .view(bsz, q_len, self.num_heads, self.head_dim)
+        .view(bsz, q_len, self.num_key_value_heads, self.head_dim)
         .transpose(1, 2)
     )
     value_states = (
-        self.v_proj(hidden_states)
+        value_states
-        .view(bsz, q_len, self.num_heads, self.head_dim)
+        .view(bsz, q_len, self.num_key_value_heads, self.head_dim)
         .transpose(1, 2)
     )
 
@@ -73,6 +94,10 @@ def xformers_forward(
 
     past_key_value = (key_states, value_states) if use_cache else None
 
+    # repeat k/v heads if n_kv_heads < n_heads
+    key_states = transformers.models.llama.modeling_llama.repeat_kv(key_states, self.num_key_value_groups)
+    value_states = transformers.models.llama.modeling_llama.repeat_kv(value_states, self.num_key_value_groups)
+
     # We only apply xformers optimizations if we don't need to output the whole attention matrix
     if not output_attentions:
         query_states = query_states.transpose(1, 2)
@@ -128,10 +153,16 @@ def xformers_forward(
                 f" {attn_output.size()}"
             )
 
-        attn_output = attn_output.transpose(1, 2)
+    attn_output = attn_output.transpose(1, 2).contiguous()
-
     attn_output = attn_output.reshape(bsz, q_len, self.hidden_size)
-    attn_output = self.o_proj(attn_output)
+
+    if self.pretraining_tp > 1:
+        attn_output = attn_output.split(self.hidden_size // self.pretraining_tp, dim=2)
+        o_proj_slices = self.o_proj.weight.split(self.hidden_size // self.pretraining_tp, dim=1)
+        attn_output = sum([F.linear(attn_output[i], o_proj_slices[i]) for i in range(self.pretraining_tp)])
+    else:
+        attn_output = self.o_proj(attn_output)    
+    
     return attn_output, attn_weights, past_key_value