Updated PyTorch ONNX exporter (markdown)

PyTorch-ONNX-exporter.md

@@ -17,6 +17,7 @@ Documentation for developing the PyTorch-ONNX exporter (`torch.onnx`).
    * [Relevant parts of PyTorch repo](#relevant-parts-of-pytorch-repo)
 * [Features](#features)
    * [Quantized model export](#quantized-model-export)
+* [Updating default opset_version](#updating-default-opset_version)
 
 # Development process
 
@@ -201,4 +202,60 @@ An example of adding unit tests for a new symbolic function: [Add binary_cross_e
 ## Quantized model export
 
 To support quantized model export, we need to unpack the quantized tensor inputs and the PackedParam weights (https://github.com/pytorch/pytorch/pull/69232). We construct through `TupleConstruct` to have a 1-to-1 input mapping, 
-so that we can use `replaceAllUsesWith` API for its successors. In addition, we support quantized namespace export, and the developers can add more symbolics for quantized operators conveniently in the current framework.
+so that we can use `replaceAllUsesWith` API for its successors. In addition, we support quantized namespace export, and the developers can add more symbolics for quantized operators conveniently in the current framework.
+
+# Updating default opset_version
+
+```python
+#!/usr/bin/env python
+
+"""Outputs what the default value of opset_version should be.
+
+The current policy is that the default should be set to the
+latest released version as of 18 months ago.
+
+Usage:
+Only argument should be path to onnx git repo.
+If run from the root of the PyTorch repo, run:
+$ <this script> third_party/onnx
+"""
+
+import datetime
+import os
+import re
+import sys
+import subprocess
+
+if len(sys.argv) != 2:
+    sys.exit("need exactly 1 argument, the path to onnx git repo")
+
+onnx_dir = sys.argv[1]
+os.chdir(onnx_dir)
+
+date = datetime.datetime.now() - datetime.timedelta(days=18*30)
+onnx_commit = subprocess.check_output(("git", "log", f"--until={date}", "--max-count=1", "--format=%H"), encoding="utf-8").strip()
+onnx_tags = subprocess.check_output(("git", "tag", "--list", f"--contains={onnx_commit}"), encoding="utf-8")
+tag_tups = []
+semver_pat = re.compile(r"v(\d+)\.(\d+)\.(\d+)")
+for tag in onnx_tags.splitlines():
+    match = semver_pat.match(tag)
+    if match:
+       tag_tups.append(tuple(int(x) for x in match.groups()))
+
+min_tup = sorted(tag_tups)[0]
+version_str = "{}.{}.{}".format(*min_tup)
+
+head_commit = subprocess.check_output(("git", "log", "--max-count=1", "--format=%H", "HEAD"), encoding="utf-8").strip()
+
+subprocess.check_call(("git", "checkout", f"v{version_str}"), stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL)
+try:
+    from onnx import helper
+    for version in helper.VERSION_TABLE:
+        if version[0] == version_str:
+            print(version[2])
+            sys.exit() # success
+    sys.exit(f"failed to find version {version_str} in onnx.helper.VERSION_TABLE at commit {onnx_commit}")
+finally:
+    subprocess.check_call(("git", "checkout", head_commit), stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL)
+
+```