Add python upgrade guide (#1826)

* Add python upgrade guide

* changes

* comments

* Update PYTHON_UPGRADE_GUIDE.MD

PYTHON_UPGRADE_GUIDE.MD

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+# Python Upgrade Runbook
+
+So you wanna upgrade PyTorch to support a new Python? Follow these steps in order!
+
+### 0.Before starting upgrade process. Check the package availability
+
+Package availability to validate before starting upgrade process :
+
+1) Manywheel build: Python version is avalilable in either of these sources:
+- Github: https://github.com/python/cpython
+- Python ftp[here](https://www.python.org/ftp/python/). Please consult the [devguide.python.org/versions](https://devguide.python.org/versions/)
+
+2) Conda build: Python package is available via anaconda [here](https://anaconda.org/anaconda/python/files). The upgrade for manywheel binaries can continue before this requirement is met.
+
+## 1. Maintain Progress and Updates
+Make an issue to track the progress, for example CUDA issue [#56721: Support 11.3](https://github.com/pytorch/pytorch/issues/56721). This is especially important as many PyTorch external users are interested in CPython upgrades.
+
+## 2. Modify scripts to install the new Python for Docker Manywheel images.
+
+1. Follow this [PR 1427](https://github.com/pytorch/builder/pull/1427) with followup [PR 1568](https://github.com/pytorch/builder/pull/1568) for all steps in this section
+2. Validate that Build manywheel docker images is successful [here](https://github.com/pytorch/builder/blob/main/.github/workflows/build-manywheel-images.yml)
+
+## 3. Modify scripts to install the new Python for Core nightyly Manywheel and Conda builds.
+
+1. Builder PRs: Follow this [PR 1586](https://github.com/pytorch/builder/pull/1586) and [PR 1593](https://github.com/pytorch/builder/pull/1593)
+2. Pytorch PRs: [PR 112882](https://github.com/pytorch/pytorch/pull/112882)
+3. Builder PR and Pytorch PR need to be tested togeather. After landing initiate nightly build and make sure nightly build is successful via [HUD](https://hud2.pytorch.org/hud/pytorch/pytorch/nightly/1?per_page=50)
+
+## 4. Land Test-Infra PRs and make sure nighly domains builds are successful
+
+1. Test-Infra PRs:  [PR 4768](https://github.com/pytorch/test-infra/pull/4768), [PR 4843](https://github.com/pytorch/test-infra/pull/4843)
+2. After landing it, intiate domain nightly builds and fix all follow up issues. Example vision [PR 8137](https://github.com/pytorch/vision/pull/8137)
+3. At this point we should have complete set of nightly builds for pytorch, vision and audio. Please validate nightly binaries with [validate-binaries workflow](https://github.com/pytorch/builder/actions/workflows/validate-binaries.yml) to make sure smoke tests are passing for Python 3.12
+
+## 5. Add support for new Pythron to CI
+
+1. Pytorch PR: [PR 117853](https://github.com/pytorch/pytorch/pull/117853)
+2. Please note, for any failures that can't be resolved easily, we ned to create new issue, and follow up on it similar to this [example](https://github.com/pytorch/pytorch/issues/120233)