store activation cls instead of function (#10832)

* store cls instead of an obj

* style

Author: SunMarc

src/diffusers/models/activations.py

@@ -24,12 +24,12 @@
 if is_torch_npu_available():
     import torch_npu
 
-ACTIVATION_FUNCTIONS = {
-    "swish": nn.SiLU(),
-    "silu": nn.SiLU(),
-    "mish": nn.Mish(),
-    "gelu": nn.GELU(),
-    "relu": nn.ReLU(),
+ACT2CLS = {
+    "swish": nn.SiLU,
+    "silu": nn.SiLU,
+    "mish": nn.Mish,
+    "gelu": nn.GELU,
+    "relu": nn.ReLU,
 }
 
 
@@ -44,10 +44,10 @@ def get_activation(act_fn: str) -> nn.Module:
     """
 
     act_fn = act_fn.lower()
-    if act_fn in ACTIVATION_FUNCTIONS:
-        return ACTIVATION_FUNCTIONS[act_fn]
+    if act_fn in ACT2CLS:
+        return ACT2CLS[act_fn]()
     else:
-        raise ValueError(f"Unsupported activation function: {act_fn}")
+        raise ValueError(f"activation function {act_fn} not found in ACT2FN mapping {list(ACT2CLS.keys())}")
 
 
 class FP32SiLU(nn.Module):