fall back to vanilla if xformers is not available (#51)

sgm/modules/diffusionmodules/model.py

@@ -1,5 +1,7 @@
 # pytorch_diffusion + derived encoder decoder
 import logging
+import warnings
+
 import math
 from typing import Any, Callable, Optional
 
@@ -291,6 +293,13 @@ def make_attn(in_channels, attn_type="vanilla", attn_kwargs=None):
             f"as it is too expensive. Please install xformers via e.g. 'pip install xformers==0.0.16'"
         )
         attn_type = "vanilla-xformers"
+    if attn_type == "vanilla-xformers" and not XFORMERS_IS_AVAILABLE:
+        warnings.warn(
+            f"Requested attention type {attn_type!r} but Xformers is not available; "
+            f"falling back to vanilla attention"
+        )
+        attn_type = "vanilla"
+        attn_kwargs = None
     logger.debug(f"making attention of type '{attn_type}' with {in_channels} in_channels")
     if attn_type == "vanilla":
         assert attn_kwargs is None