Revert "fall back to vanilla if xformers is not available (#51)" (#61)

This reverts commit ef520df1db.

sgm/modules/diffusionmodules/model.py

@@ -1,7 +1,5 @@
 # pytorch_diffusion + derived encoder decoder
 import logging
-import warnings
-
 import math
 from typing import Any, Callable, Optional
 
@@ -293,13 +291,6 @@ 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