[PyTorch] Fix multiple calls to saved_tensors in CP attention (#1334)

* Limit to one call of ctx.saved_tensors per autograd bwd

Signed-off-by: Kirthi Shankar Sivamani <ksivamani@nvidia.com>

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

---------

Signed-off-by: Kirthi Shankar Sivamani <ksivamani@nvidia.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

transformer_engine/pytorch/attention.py

@@ -2528,12 +2528,13 @@ def backward(ctx, dout):
         recv_src = ctx.cp_global_ranks[(rank + 1) % cp_size * cp_size_a2a + rank_a2a]
         batch_p2p_comm = int(os.getenv("NVTE_BATCH_MHA_P2P_COMM", "0")) or (cp_size == 2)
 
-        (q, kv, out, softmax_lse, cu_seqlens_q_padded, cu_seqlens_kv_padded) = ctx.saved_tensors[:6]
-        (fp8_fwd_scales, fp8_fwd_scale_invs) = ctx.saved_tensors[6:8]
-        cu_seqlens_q_per_step = ctx.saved_tensors[8 : 8 + cp_size]
-        cu_seqlens_kv_per_step = ctx.saved_tensors[8 + cp_size : 8 + cp_size * 2]
-        rng_states = ctx.saved_tensors[8 + cp_size * 2 : 8 + cp_size * 3]
-        attn_biases = ctx.saved_tensors[8 + cp_size * 3 : 8 + cp_size * 4]
+        (*saved_tensors,) = ctx.saved_tensors
+        (q, kv, out, softmax_lse, cu_seqlens_q_padded, cu_seqlens_kv_padded) = saved_tensors[:6]
+        (fp8_fwd_scales, fp8_fwd_scale_invs) = saved_tensors[6:8]
+        cu_seqlens_q_per_step = saved_tensors[8 : 8 + cp_size]
+        cu_seqlens_kv_per_step = saved_tensors[8 + cp_size : 8 + cp_size * 2]
+        rng_states = saved_tensors[8 + cp_size * 2 : 8 + cp_size * 3]
+        attn_biases = saved_tensors[8 + cp_size * 3 : 8 + cp_size * 4]
 
         causal = "causal" in ctx.attn_mask_type
         padding = "padding" in ctx.attn_mask_type
@@ -3577,11 +3578,12 @@ def backward(ctx, dout):
         cp_size = get_distributed_world_size(ctx.cp_group)
         rank = get_distributed_rank(ctx.cp_group)
 
-        (q, k, v, cu_seqlens_q, cu_seqlens_q_padded) = ctx.saved_tensors[:5]
-        cu_seqlens_kv_per_step = ctx.saved_tensors[5:7]
-        out_per_step = ctx.saved_tensors[7:9]
-        softmax_lse_per_step = ctx.saved_tensors[9:11]
-        rng_states = ctx.saved_tensors[11:13]
+        (*saved_tensors,) = ctx.saved_tensors
+        (q, k, v, cu_seqlens_q, cu_seqlens_q_padded) = saved_tensors[:5]
+        cu_seqlens_kv_per_step = saved_tensors[5:7]
+        out_per_step = saved_tensors[7:9]
+        softmax_lse_per_step = saved_tensors[9:11]
+        rng_states = saved_tensors[11:13]
         kv_seq_range_per_step = ctx.kv_seq_range_per_step
         window_size_per_step = ctx.window_size_per_step
 
@@ -4056,12 +4058,11 @@ def backward(ctx, dout):
         # pylint: disable=missing-function-docstring
         cp_size = get_distributed_world_size(ctx.cp_group)
 
-        q, k, v, out = ctx.saved_tensors[:4]
-        cu_seqlens_q, cu_seqlens_kv, cu_seqlens_q_padded, cu_seqlens_kv_padded = ctx.saved_tensors[
-            4:8
-        ]
-        fp8_fwd_scales, fp8_fwd_scale_invs = ctx.saved_tensors[8:10]
-        aux_ctx_tensors = ctx.saved_tensors[10:]
+        (*saved_tensors,) = ctx.saved_tensors
+        q, k, v, out = saved_tensors[:4]
+        cu_seqlens_q, cu_seqlens_kv, cu_seqlens_q_padded, cu_seqlens_kv_padded = saved_tensors[4:8]
+        fp8_fwd_scales, fp8_fwd_scale_invs = saved_tensors[8:10]
+        aux_ctx_tensors = saved_tensors[10:]
 
         qkv_layout = ctx.qkv_format + "_" + ctx.qkv_format + "_" + ctx.qkv_format
         causal = "causal" in ctx.attn_mask_type