From d8d440db94e4399016661de31e152bfe47ad6d52 Mon Sep 17 00:00:00 2001
From: kaixih <kaixih@nvidia.com>
Date: Wed, 13 Mar 2024 17:09:42 +0000
Subject: [PATCH] Add cudnn attention layer

---
 praxis/layers/gpu_fast_attention.py | 80 +++++++++++++++++++++++++++++
 1 file changed, 80 insertions(+)

diff --git a/praxis/layers/gpu_fast_attention.py b/praxis/layers/gpu_fast_attention.py
index cc066964..b29059dc 100644
--- a/praxis/layers/gpu_fast_attention.py
+++ b/praxis/layers/gpu_fast_attention.py
@@ -25,6 +25,8 @@
 
 import jax
 from jax.experimental.shard_map import shard_map
+import numpy as np
+
 from praxis import asserts
 from praxis import base_layer
 from praxis import py_utils
@@ -46,6 +48,84 @@
 
 JTensor = pytypes.JTensor
 
+class GpuCudnnFusedDotProductAttention(attentions.DotProductAttention):
+  """Using Jax/Cudnn to call into a fused MHA kernel on NVIDIA GPU."""
+  is_causal: bool = False
+
+  def _shard_only_bn(self, x: JTensor) -> JTensor:
+    """Adds sharding annotations to tensors of shape [b, n, None, None]."""
+    ap = self.activation_split_dims_mapping
+    if self.mesh_axis_names is None or ap.blnh is None:
+      return x
+    assert len(ap.blnh) == 4
+    b = [ap.blnh[0], ap.blnh[2], None, None]
+    return base_layer.maybe_shard(x, b, self.mesh_axis_names)
+
+  def _dot_atten(
+      self,
+      query: JTensor,
+      key: JTensor,
+      value: JTensor,
+      atten_mask: JTensor,
+      relative_bias: JTensor | None = None,
+  ) -> tuple[JTensor, JTensor]:
+    """Main attention function.
+
+    Args:
+      query: JTensor of shape [B, T, N, H].
+      key: JTensor of shape [B, S, N, H].
+      value: JTensor of shape [B, S, N, H].
+      atten_mask: JTensor of shape [1|B, 1, 1|T, S] which is a mask that is
+        applied to prevent attention between unwanted pairs. This has already
+        been converted into large negative logits. Note that the first and third
+        dimension allow size 1 if the mask is shared by every item in the batch
+        or every token in the target sequence.
+      relative_bias: Relative bias of shape [B, N, T, S].
+
+    Returns:
+      encoded: JTensor of shape [B, T, N, H].
+      atten_probs: JTensor of shape [B, N, T, S].
+    """
+    query = self._shard_blnh(query)
+    key = self._shard_blnh(key)
+    value = self._shard_blnh(value)
+
+    b, s, n, h = key.shape
+    base_layer.assert_has_shape(value, [b, s, n, h])
+    base_layer.assert_has_shape(query, [b, -1, n, h])
+    t = query.shape[1]
+    # If only padding bias is supplied, then atten_mask can be [B, 1, 1, S]
+    # since each target token is prohibited from attending to the same set of
+    # source tokens. In this case tiling is inefficient and unnecessary.
+    # If there is no padding mask, and only causal mask then the shape can be
+    # [1, 1, T, S]
+    base_layer.assert_has_shape(atten_mask, [-1, 1, -1, s])
+    asserts.in_set(atten_mask.shape[2], [t, 1])
+    asserts.in_set(atten_mask.shape[0], [b, 1])
+
+    assert self.attention_extra_logit is None
+    assert not self.zero_fully_masked
+    assert not self.atten_logit_cap or self.atten_logit_cap <= 0.0
+
+    if self.atten_dropout_prob > 0.0 and not self.do_eval:
+      raise NotImplementedError
+    
+    query = self._scale_query(query)
+    logits_scale = 1.0 / np.sqrt(h) if self.scale_logits_by_head_dims else 1.0
+
+    # Explicitly shard the relative_bias to ensure it has the same sharding on
+    # batch and num_head dim with the query. This is required by the
+    # dot_product_attention.
+    if relative_bias is not None:
+      relative_bias = self._shard_only_bn(relative_bias)
+
+    encoded = jax.nn.dot_product_attention(
+        query, key, value, bias=relative_bias, scale=logits_scale,
+        is_causal=self.is_causal, implementation='cudnn',
+    )
+    encoded = self._shard_blnh(encoded)
+    return encoded, None
+
 
 class GpuTritonFusedDotProductAttention(attentions.DotProductAttention):
   """Using Jax/Pallas/Triton to call into a fused MHA kernel on NVIDIA GPU."""