add TransformerLens example (#2651)

* add TransformerLens example

Many people use TransformerLens to do interpretability and interventions on models, and then need to test the model.

Here is a simple script that allows one to pass in the TransformerLens model and run evaluations on it.

* Ran pre-commit checks

examples/transformer-lens.py

+import warnings
+
+import torch
+import torch.nn as nn
+from transformer_lens import HookedTransformer
+from transformers import AutoConfig
+
+from lm_eval import evaluator
+from lm_eval.models.huggingface import HFLM
+
+
+def evaluate_lm_eval(lens_model: HookedTransformer, tasks: list[str], **kwargs):
+    class HFLikeModelAdapter(nn.Module):
+        """Adapts HookedTransformer to match the HuggingFace interface expected by lm-eval"""
+
+        def __init__(self, model: HookedTransformer):
+            super().__init__()
+            self.model = model
+            self.tokenizer = model.tokenizer
+            self.config = AutoConfig.from_pretrained(model.cfg.tokenizer_name)
+            self.device = model.cfg.device
+            self.tie_weights = lambda: self
+
+        def forward(self, input_ids=None, attention_mask=None, **kwargs):
+            output = self.model(input_ids, attention_mask=attention_mask, **kwargs)
+            # Make sure output has the expected .logits attribute
+            if not hasattr(output, "logits"):
+                if isinstance(output, torch.Tensor):
+                    output.logits = output
+            return output
+
+        # Only delegate specific attributes we know we need
+        def to(self, *args, **kwargs):
+            return self.model.to(*args, **kwargs)
+
+        def eval(self):
+            self.model.eval()
+            return self
+
+        def train(self, mode=True):
+            self.model.train(mode)
+            return self
+
+    model = HFLikeModelAdapter(lens_model)
+    warnings.filterwarnings("ignore", message="Failed to get model SHA for")
+    results = evaluator.simple_evaluate(
+        model=HFLM(pretrained=model, tokenizer=model.tokenizer),
+        tasks=tasks,
+        verbosity="WARNING",
+        **kwargs,
+    )
+    return results
+
+
+if __name__ == "__main__":
+    # Load base model
+    model = HookedTransformer.from_pretrained("pythia-70m")
+    res = evaluate_lm_eval(model, tasks=["arc_easy"])
+    print(res["results"])