Model: Fix flash-attn checks

If flash attention is already turned off by exllamaV2 itself, don't
try creating a paged generator. Also condense all the redundant
logic into one if statement.

Also check arch_compat_overrides to see if flash attention should
be disabled for a model arch (ex. Gemma 2)

Signed-off-by: kingbri <bdashore3@proton.me>

backends/exllamav2/utils.py

@@ -94,3 +94,18 @@ def supports_paged_attn():
         return False
     else:
         return True
+
+
+def exllama_disabled_flash_attn(no_flash_attn: bool):
+    unsupported_message = (
+        "ExllamaV2 has disabled Flash Attention. \n"
+        "Please see the above logs for warnings/errors. \n"
+        "Switching to compatibility mode. \n"
+        "This disables parallel batching "
+        "and features that rely on it (ex. CFG). \n"
+    )
+
+    if no_flash_attn:
+        logger.warning(unsupported_message)
+
+    return no_flash_attn