Updated comment

src/transformers/generation/utils.py

@@ -381,9 +381,11 @@ def prepare_inputs_for_generation(
         # Exception 2: some generation methods do special slicing of input_ids, so we don't need to do it here
         # Exception 3: with synced GPUs cache_position may go out of bounds, but we only want dummy token in that case.
         #              (we can't check exception 3 while compiling)
+        # Excpetion 4: If input_embeds are passed then slice it through `cache_position`, to keep only the unprocessed tokens and
+        # generate the first token for each sequence. Later use the generated Input ids for continuation.
         if past_key_values is not None:
             model_inputs["past_key_values"] = past_key_values
-            if inputs_embeds is not None and input_ids.shape[1] == 0:
+            if inputs_embeds is not None and input_ids.shape[1] == 0:  # Exception 4
                 inputs_embeds = inputs_embeds[:, -cache_position.shape[0] :]
             elif (
                 inputs_embeds is not None  # Exception 1