Prevent config script from trying to set vram on macs (#4412)

## What type of PR is this? (check all applicable)

- [X] Bug Fix

## Have you discussed this change with the InvokeAI team?
- [X] Yes
      
## Have you updated all relevant documentation?
- [X] Yes


## Description

Running the config script on Macs triggered an error due to absence of
VRAM on these machines! VRAM setting is now skipped.

## Added/updated tests?

- [ ] Yes
- [X] No : Will add this test in the near future.

Author: lstein

invokeai/backend/install/invokeai_configure.py

@@ -507,7 +507,7 @@ def create(self):
                 scroll_exit=True,
             )
         else:
-            self.vram_cache_size = DummyWidgetValue.zero
+            self.vram = DummyWidgetValue.zero
         self.nextrely += 1
         self.outdir = self.add_widget_intelligent(
             FileBox,
@@ -605,7 +605,8 @@ def marshall_arguments(self):
             "vram",
             "outdir",
         ]:
-            setattr(new_opts, attr, getattr(self, attr).value)
+            if hasattr(self, attr):
+                setattr(new_opts, attr, getattr(self, attr).value)
 
         for attr in self.autoimport_dirs:
             directory = Path(self.autoimport_dirs[attr].value)