better error message (#5497)

src/transformers/generation_utils.py

@@ -428,6 +428,10 @@ class GenerationMixin:
             encoder_outputs = None
             cur_len = input_ids.shape[-1]
 
+        assert (
+            cur_len < max_length
+        ), f"The context has {cur_len} number of tokens, but `max_length` is only {max_length}. Please make sure that `max_length` is bigger than the number of tokens, by setting either `generate(max_length=...,...)` or `config.max_length = ...`"
+
         if num_beams > 1:
             output = self._generate_beam_search(
                 input_ids,