[draft_export] add LOC for data-dep error logging (#145443)

Summary:
maybe this is too much info, but it's difficult to go through old draft export reports where the stack trace is out of sync with the current codebase. Data-dependent errors now look like:
```
2. Data dependent error.
    When exporting, we were unable to evaluate the value of `u306`.
    This occurred at the following stacktrace:
    File /data/users/pianpwk/fbsource/buck-out/v2/gen/fbcode/78204cab86e8a0fb/sigmoid/inference/ts_migration/__pt2i_readiness_main__/pt2i_readiness_main#link-tree/caffe2/torch/fb/training_toolkit/common/proxy_module_thrift/embedding_bag_proxy.py, lineno 109, in _forward_impl:
         `if offsets[-1] > len(input):`
    As a result, it was specialized to evaluate to `261`, and asserts were inserted into the graph.
    Please add `torch._check(...)` to the original code to assert this data-dependent assumption.
    Please refer to https://docs.google.com/document/d/1kZ_BbB3JnoLbUZleDT6635dHs88ZVYId8jT-yTFgf3A/edit#heading=h.boi2xurpqa0o for more details.
```

This would be even more helpful for reports on torch-packaged models, but that requires some more work on PT2I-specific stack trace processing

Test Plan: .

Differential Revision: D68534017

Pull Request resolved: https://github.com/pytorch/pytorch/pull/145443
Approved by: https://github.com/angelayi

torch/export/_draft_export.py

@@ -55,6 +55,17 @@ def hash_stack(stack: list[dict[str, str]]) -> str:
     return ";".join(f'line: {s["line"]} filename: {s["filename"]}' for s in stack)
 
 
+def get_loc(filename: str, lineno: int) -> Optional[str]:
+    try:
+        with open(filename) as f:
+            for i, line in enumerate(f):
+                if i == lineno - 1:
+                    return line.strip()
+    except FileNotFoundError:
+        pass
+    return None
+
+
 class FailureReport:
     def __init__(
         self, failure_type: FailureType, data: dict[str, Any], xfail: bool = False
@@ -90,10 +101,18 @@ class FailureReport:
 """
 
         elif self.failure_type == FailureType.DATA_DEPENDENT_ERROR:
+            loc = None
+            if self.data["stack"]:
+                frame = self.data["stack"][-1]
+                loc = (
+                    f"`{get_loc(str_to_filename[frame['filename']], frame['line'])}`"
+                    or ""
+                )
             return f"""Data dependent error.
-    When exporting, we were unable to figure out if the expression `{self.data["expr"]}` always holds.
+    When exporting, we were unable to evaluate the value of `{self.data["expr"]}`.
     This was encountered {self.data["occurrences"]} times.
-    This occurred at the following stacktrace: {prettify_stack(self.data["stack"], str_to_filename)}.
+    This occurred at the following stacktrace: {prettify_stack(self.data["stack"], str_to_filename)}:
+        {loc}
     As a result, it was specialized to a constant (e.g. `{self.data["result"]}` in the 1st occurrence), and asserts were inserted into the graph.
 
     Please add `torch._check(...)` to the original code to assert this data-dependent assumption.