Fixes PYTORCH_ENABLE_MPS_FALLBACK not set correctly (#2508)

`torch` wasn't seeing the environment variable. I suspect this is
because it was imported before the variable was set, so was running with
a different environment.

Many `torch` ops are supported on MPS so this wasn't noticed
immediately, but some samplers like k_dpm_2 still use unsupported
operations and need this fallback.

ldm/invoke/CLI.py

@@ -4,6 +4,9 @@ import sys
 import shlex
 import traceback
 
+if sys.platform == "darwin":
+    os.environ["PYTORCH_ENABLE_MPS_FALLBACK"] = "1"
+
 from ldm.invoke.globals import Globals
 from ldm.generate import Generate
 from ldm.invoke.prompt_parser import PromptParser
@@ -21,9 +24,6 @@ import ldm.invoke
 # global used in multiple functions (fix)
 infile = None
 
-if sys.platform == 'darwin':
-    os.environ["PYTORCH_ENABLE_MPS_FALLBACK"] = "1"
-
 def main():
     """Initialize command-line parsers and the diffusion model"""
     global infile

scripts/invoke.py

@@ -1,7 +1,4 @@
 #!/usr/bin/env python
 
-import sys
-import os
-
 import ldm.invoke.CLI
 ldm.invoke.CLI.main()