Remove multiple if-else statement in the get_activation function. (#5446)

* I added a new doc string to the class. This is more flexible to understanding other developers what are doing and where it's using.

* Update src/diffusers/models/unet_2d_blocks.py

This changes suggest by maintener.
Co-authored-by: Sayak Paul <spsayakpaul@gmail.com>

* Update src/diffusers/models/unet_2d_blocks.py

Add suggested text
Co-authored-by: Sayak Paul <spsayakpaul@gmail.com>

* Update unet_2d_blocks.py

I changed the Parameter to Args text.

* Update unet_2d_blocks.py

proper indentation set in this file.

* Update unet_2d_blocks.py

a little bit of change in the act_fun argument line.

* I run the black command to reformat style in the code

* Update unet_2d_blocks.py

similar doc-string add to have in the original diffusion repository.

* I use a lower method in the activation function.

* Replace multiple if-else statements with a dictionary of activation functions, and call one if statement to retrieve the appropriate function.

* I am using black package to reforamted my file

* I defined the ACTIVATION_FUNCTIONS variable outside of the function

* activation function variable convert to lower case

* First, I resolved the conflict issue. Then, I ran the Black package to reformat my file.

---------
Co-authored-by: Sayak Paul <spsayakpaul@gmail.com>

src/diffusers/models/activations.py

@@ -21,6 +21,15 @@ from ..utils import USE_PEFT_BACKEND
 from .lora import LoRACompatibleLinear
 
 
+ACTIVATION_FUNCTIONS = {
+    "swish": nn.SiLU(),
+    "silu": nn.SiLU(),
+    "mish": nn.Mish(),
+    "gelu": nn.GELU(),
+    "relu": nn.ReLU(),
+}
+
+
 def get_activation(act_fn: str) -> nn.Module:
     """Helper function to get activation function from string.
 
@@ -30,14 +39,10 @@ def get_activation(act_fn: str) -> nn.Module:
     Returns:
         nn.Module: Activation function.
     """
-    if act_fn in ["swish", "silu"]:
-        return nn.SiLU()
-    elif act_fn == "mish":
-        return nn.Mish()
-    elif act_fn == "gelu":
-        return nn.GELU()
-    elif act_fn == "relu":
-        return nn.ReLU()
+
+    act_fn = act_fn.lower()
+    if act_fn in ACTIVATION_FUNCTIONS:
+        return ACTIVATION_FUNCTIONS[act_fn]
     else:
         raise ValueError(f"Unsupported activation function: {act_fn}")