Model: Robust request length checking in generator

* Ensure that length of positive/negative prompt + max_tokens does not exceed max_seq_len
* Ensure that total required pages for CFG request does not exceed allocated cache_size

backends/exllamav2/model.py

@@ -1301,17 +1301,51 @@ class ExllamaV2Container:
 
         # The first index will always be the positive prompt
         context_len = input_ids[0].size(dim=-1)
-        if context_len > self.config.max_seq_len:
-            raise ValueError(
-                f"Context length {context_len} is greater than max_seq_len "
-                f"{self.config.max_seq_len}"
-            )
+
+        # The second index will be the negative prompt if CFG is enabled
+        if negative_prompt is not None:
+            negative_context_len = input_ids[1].size(dim=-1)
+        else:
+            negative_context_len = 0
 
         # Automatically set max_tokens to fill up the context
         # This should be an OK default, but may be changed in the future
         max_tokens = unwrap(
-            kwargs.get("max_tokens"), self.config.max_seq_len - context_len
+            kwargs.get("max_tokens"),
+            self.config.max_seq_len - max(context_len, negative_context_len),
         )
+        if max_tokens < 1:
+            logger.warning("max_tokens must be a positive integer, " "setting to 1.")
+            max_tokens = 1
+
+        # Check total length of request
+        if context_len + max_tokens > self.config.max_seq_len:
+            raise ValueError(
+                f"Request length {context_len} + {max_tokens} is greater than "
+                f"max_seq_len {self.config.max_seq_len}"
+            )
+
+        # Check total length of negative prompt request if CFG is enabled
+        if negative_prompt is not None:
+            if context_len + max_tokens > self.config.max_seq_len:
+                raise ValueError(
+                    f"Request length for negative prompt "
+                    f"{negative_context_len} + {max_tokens} is greater than "
+                    f"max_seq_len {self.config.max_seq_len}"
+                )
+            # Check total required pages for CFG request
+            if (
+                sum(
+                    256 * math.ceil((context + max_tokens) / 256)
+                    for context in (context_len, negative_context_len)
+                )
+                > self.cache_size
+            ):
+                raise ValueError(
+                    f"Total required page size for request "
+                    f"{context_len} + {negative_context_len} + {max_tokens} * 2 "
+                    f"is greater than cache_size {self.cache_size}"
+                )
 
         # Set min_tokens to generate while keeping EOS banned
         min_tokens = unwrap(kwargs.get("min_tokens"), 0)