community[patch]: huggingface hub character removal bug fix (#16233)

- **Description:** Some text-generation models on huggingface repeat the
prompt in their generated response, but not all do! The tests use "gpt2"
which DOES repeat the prompt and as such, the HuggingFaceHub class is
hardcoded to remove the first few characters of the response (to match
the len(prompt)). However, if you are using a model (such as the very
popular "meta-llama/Llama-2-7b-chat-hf") that DOES NOT repeat the prompt
in it's generated text, then the beginning of the generated text will be
cut off. This code change fixes that bug by first checking whether the
prompt is repeated in the generated response and removing it
conditionally.
  - **Issue:** #16232 
  - **Dependencies:** N/A
  - **Twitter handle:** N/A

libs/community/langchain_community/llms/huggingface_hub.py

@@ -112,8 +112,10 @@ class HuggingFaceHub(LLM):
         if "error" in response:
             raise ValueError(f"Error raised by inference API: {response['error']}")
         if self.client.task == "text-generation":
-            # Text generation return includes the starter text.
-            text = response[0]["generated_text"][len(prompt) :]
+            # Text generation sometimes return includes the starter text.
+            text = response[0]["generated_text"]
+            if text.startswith(prompt):
+                text = response[0]["generated_text"][len(prompt) :]
         elif self.client.task == "text2text-generation":
             text = response[0]["generated_text"]
         elif self.client.task == "summarization":