Fix GPT2 impl partially

TODO: still need to add `until` everywhere

lm_eval/models/gpt2.py

@@ -17,19 +17,22 @@ class GPT2LM(LM):
         return cls(device=args.get("device", "cpu"))
 
     def generate(self, context, max_gen_length, truncate=True):
-        context_tensor = torch.tensor([self.tokenizer.encode(context.strip())], dtype=torch.long).to(self.device)
+        # when too long to fit in context, truncate from the left
+        context_tensor = torch.tensor([self.tokenizer.encode(context.strip())[max_gen_length - 1024:]], dtype=torch.long).to(self.device)
         res = self.gpt2.generate(
             context_tensor,
+            # TODO: change to have until rather than using eos_token_id
             eos_token_id=self.tokenizer.eos_token_id,
             do_sample=False,
             max_length=self.num_tokens(context) + max_gen_length,
         )
 
         # chop off the prompt and the final eos token
-        return self.tokenizer.decode(res[0][len(context[0]):-1]).strip()
+        return self.tokenizer.decode(res[0][min(1024 - max_gen_length, len(context_tensor[0])):-1]).strip()
 
     def loglikelihood(self, context, continuation, truncate=True):
-        inp = torch.tensor([self.tokenizer.encode(context + continuation)], dtype=torch.long).to(self.device)
+        # when too long to fit in context, truncate from the left
+        inp = torch.tensor([self.tokenizer.encode(context + continuation)[-1024:]], dtype=torch.long).to(self.device)
         ctxlen = len(self.tokenizer.encode(context.strip()))
 
         cont_toks = inp[:, ctxlen:]  # [batch, seq]