fix: only instantiate CrossAttentionBlock when with_cross_attention=True

[200~Fixes #8845

  TransformerBlock previously instantiated norm_cross_attn and cross_attn
  unconditionally, even when with_cross_attention=False. These unused modules
  registered dead parameters in model.parameters(), wasting memory.

  Wrapped both instantiations in `if with_cross_attention:` to match the
  existing guard in forward(). Added tests to verify the modules and their
  parameters are absent when disabled, present when enabled, and that the
  forward pass with a context tensor works correctly.~

Signed-off-by: chhayankjain <chhayank44@gmail.com>

Author: chhayankjain

monai/networks/blocks/transformerblock.py

@@ -46,11 +46,20 @@ def __init__(
             dropout_rate (float, optional): fraction of the input units to drop. Defaults to 0.0.
             qkv_bias(bool, optional): apply bias term for the qkv linear layer. Defaults to False.
             save_attn (bool, optional): to make accessible the attention matrix. Defaults to False.
+            causal (bool, optional): whether to apply causal masking in self-attention. Defaults to False.
+            sequence_length (int | None, optional): sequence length required for causal masking. Defaults to None.
+            with_cross_attention (bool, optional): whether to include cross-attention layers that attend to an
+                external context tensor. When False, cross_attn is set to nn.Identity() so that the attribute
+                always exists for typing and checkpoint compatibility. Defaults to False.
             use_flash_attention: if True, use Pytorch's inbuilt flash attention for a memory efficient attention mechanism
                 (see https://pytorch.org/docs/2.2/generated/torch.nn.functional.scaled_dot_product_attention.html).
             include_fc: whether to include the final linear layer. Default to True.
             use_combined_linear: whether to use a single linear layer for qkv projection, default to True.
 
+        Raises:
+            ValueError: if dropout_rate is not in [0, 1].
+            ValueError: if hidden_size is not divisible by num_heads.
+
         """
 
         super().__init__()
@@ -79,14 +88,18 @@ def __init__(
         self.with_cross_attention = with_cross_attention
 
         self.norm_cross_attn = nn.LayerNorm(hidden_size)
-        self.cross_attn = CrossAttentionBlock(
-            hidden_size=hidden_size,
-            num_heads=num_heads,
-            dropout_rate=dropout_rate,
-            qkv_bias=qkv_bias,
-            causal=False,
-            use_flash_attention=use_flash_attention,
-        )
+        self.cross_attn: CrossAttentionBlock | nn.Identity
+        if with_cross_attention:
+            self.cross_attn = CrossAttentionBlock(
+                hidden_size=hidden_size,
+                num_heads=num_heads,
+                dropout_rate=dropout_rate,
+                qkv_bias=qkv_bias,
+                causal=False,
+                use_flash_attention=use_flash_attention,
+            )
+        else:
+            self.cross_attn = nn.Identity()
 
     def forward(
         self, x: torch.Tensor, context: torch.Tensor | None = None, attn_mask: torch.Tensor | None = None

monai/networks/nets/transformer.py

@@ -147,7 +147,7 @@ def load_old_state_dict(self, old_state_dict: dict, verbose=False) -> None:
 
         # fix the renamed norm blocks first  norm2 -> norm_cross_attention , norm3 -> norm2
         for k in list(old_state_dict.keys()):
-            if "norm2" in k:
+            if "norm2" in k and k.replace("norm2", "norm_cross_attn") in new_state_dict:
                 new_state_dict[k.replace("norm2", "norm_cross_attn")] = old_state_dict.pop(k)
             if "norm3" in k:
                 new_state_dict[k.replace("norm3", "norm2")] = old_state_dict.pop(k)

tests/networks/blocks/test_transformerblock.py

@@ -16,9 +16,11 @@
 
 import numpy as np
 import torch
+import torch.nn as nn
 from parameterized import parameterized
 
 from monai.networks import eval_mode
+from monai.networks.blocks.crossattention import CrossAttentionBlock
 from monai.networks.blocks.transformerblock import TransformerBlock
 from monai.utils import optional_import
 from tests.test_utils import dict_product
@@ -53,6 +55,36 @@ def test_ill_arg(self):
         with self.assertRaises(ValueError):
             TransformerBlock(hidden_size=622, num_heads=8, mlp_dim=3072, dropout_rate=0.4)
 
+    @skipUnless(has_einops, "Requires einops")
+    def test_cross_attention_is_identity_when_disabled(self):
+        block = TransformerBlock(hidden_size=128, mlp_dim=256, num_heads=4, with_cross_attention=False)
+        # attributes always exist for typing and checkpoint compatibility
+        self.assertTrue(hasattr(block, "cross_attn"))
+        self.assertTrue(hasattr(block, "norm_cross_attn"))
+        # cross_attn is nn.Identity (no parameters) when disabled
+        self.assertIsInstance(block.cross_attn, nn.Identity)
+        param_names = [name for name, _ in block.named_parameters()]
+        self.assertFalse(any(n.startswith("cross_attn") for n in param_names))
+
+    @skipUnless(has_einops, "Requires einops")
+    def test_cross_attention_params_registered_when_enabled(self):
+        block = TransformerBlock(hidden_size=128, mlp_dim=256, num_heads=4, with_cross_attention=True)
+        self.assertIsInstance(block.cross_attn, CrossAttentionBlock)
+        self.assertTrue(hasattr(block, "norm_cross_attn"))
+        param_names = [name for name, _ in block.named_parameters()]
+        self.assertTrue(any("cross_attn" in n for n in param_names))
+        self.assertTrue(any("norm_cross_attn" in n for n in param_names))
+
+    @skipUnless(has_einops, "Requires einops")
+    def test_cross_attention_forward_with_context(self):
+        hidden_size = 128
+        block = TransformerBlock(hidden_size=hidden_size, mlp_dim=256, num_heads=4, with_cross_attention=True)
+        x = torch.randn(2, 16, hidden_size)
+        context = torch.randn(2, 8, hidden_size)
+        with eval_mode(block):
+            out = block(x, context=context)
+        self.assertEqual(out.shape, x.shape)
+
     @skipUnless(has_einops, "Requires einops")
     def test_access_attn_matrix(self):
         # input format