[Easy] Add notes for setting up dev venv with specific Python version (#164214)

Resolves https://github.com/pytorch/pytorch/issues/164010#issuecomment-3340751377

Pull Request resolved: https://github.com/pytorch/pytorch/pull/164214
Approved by: https://github.com/ezyang
ghstack dependencies: #162324

CONTRIBUTING.md

@@ -81,7 +81,7 @@ git remote add upstream git@github.com:pytorch/pytorch.git
 make setup-env
 # Or run `make setup-env-cuda` for pre-built CUDA binaries
 # Or run `make setup-env-rocm` for pre-built ROCm binaries
-source venv/bin/activate  # or `& .\venv\Scripts\Activate.ps1` on Windows
+source venv/bin/activate  # or `. .\venv\Scripts\activate` on Windows
 ```
 
 ### Tips and Debugging
@@ -182,28 +182,36 @@ You can use this script to check out a new nightly branch with the following:
 
 ```bash
 ./tools/nightly.py checkout -b my-nightly-branch
-source venv/bin/activate  # or `& .\venv\Scripts\Activate.ps1` on Windows
+source venv/bin/activate  # or `. .\venv\Scripts\activate` on Windows
 ```
 
 To install the nightly binaries built with CUDA, you can pass in the flag `--cuda`:
 
 ```bash
 ./tools/nightly.py checkout -b my-nightly-branch --cuda
-source venv/bin/activate  # or `& .\venv\Scripts\Activate.ps1` on Windows
+source venv/bin/activate  # or `. .\venv\Scripts\activate` on Windows
 ```
 
 To install the nightly binaries built with ROCm, you can pass in the flag `--rocm`:
 
 ```bash
 ./tools/nightly.py checkout -b my-nightly-branch --rocm
-source venv/bin/activate  # or `& .\venv\Scripts\Activate.ps1` on Windows
+source venv/bin/activate  # or `. .\venv\Scripts\activate` on Windows
 ```
 
 You can also use this tool to pull the nightly commits into the current branch:
 
 ```bash
-./tools/nightly.py pull -p my-env
-source my-env/bin/activate  # or `& .\venv\Scripts\Activate.ps1` on Windows
+./tools/nightly.py pull
+source venv/bin/activate  # or `. .\venv\Scripts\activate` on Windows
+```
+
+To create the virtual environment with a specific Python interpreter, you can
+pass in the `--python` argument:
+
+```bash
+./tools/nightly.py --python /path/to/python3.12
+source venv/bin/activate  # or `. .\venv\Scripts\activate` on Windows
 ```
 
 Pulling will recreate a fresh virtual environment and reinstall the development

tools/nightly.py

@@ -7,12 +7,12 @@ binaries into the repo.
 You can use this script to check out a new nightly branch with the following::
 
     $ ./tools/nightly.py checkout -b my-nightly-branch
-    $ source venv/bin/activate  # or `& .\venv\Scripts\Activate.ps1` on Windows
+    $ source venv/bin/activate  # or `. .\venv\Scripts\activate` on Windows
 
 Or if you would like to check out the nightly commit in detached HEAD mode::
 
     $ ./tools/nightly.py checkout
-    $ source venv/bin/activate  # or `& .\venv\Scripts\Activate.ps1` on Windows
+    $ source venv/bin/activate  # or `. .\venv\Scripts\activate` on Windows
 
 Or if you would like to reuse an existing virtual environment, you can pass in
 the prefix argument (--prefix)::
@@ -23,18 +23,24 @@ the prefix argument (--prefix)::
 To install the nightly binaries built with CUDA, you can pass in the flag --cuda::
 
     $ ./tools/nightly.py checkout -b my-nightly-branch --cuda
-    $ source venv/bin/activate  # or `& .\venv\Scripts\Activate.ps1` on Windows
+    $ source venv/bin/activate  # or `. .\venv\Scripts\activate` on Windows
 
 To install the nightly binaries built with ROCm, you can pass in the flag --rocm::
 
     $ ./tools/nightly.py checkout -b my-nightly-branch --rocm
-    $ source venv/bin/activate  # or `& .\venv\Scripts\Activate.ps1` on Windows
+    $ source venv/bin/activate  # or `. .\venv\Scripts\activate` on Windows
 
 You can also use this tool to pull the nightly commits into the current branch as
 well. This can be done with::
 
     $ ./tools/nightly.py pull
-    $ source venv/bin/activate  # or `& .\venv\Scripts\Activate.ps1` on Windows
+    $ source venv/bin/activate  # or `. .\venv\Scripts\activate` on Windows
+
+To create the virtual environment with a specific Python interpreter, you can
+pass in the --python argument::
+
+    $ ./tools/nightly.py --python /path/to/python3.12
+    $ source venv/bin/activate  # or `. .\venv\Scripts\activate` on Windows
 
 Pulling will recreate a fresh virtual environment and reinstall the development
 dependencies as well as the nightly binaries into the repo directory.
@@ -308,7 +314,7 @@ class Venv:
         """Get the activation script for the virtual environment."""
         if WINDOWS:
             # Assume PowerShell
-            return self.prefix / "Scripts" / "Activate.ps1"
+            return self.prefix / "Scripts" / "activate"
         # Assume POSIX-compliant shell: Bash, Zsh, etc.
         return self.prefix / "bin" / "activate"
 
@@ -317,7 +323,7 @@ class Venv:
         """Get the command to activate the virtual environment."""
         if WINDOWS:
             # Assume PowerShell
-            return f'& "{self.activate_script}"'
+            return f'. "{self.activate_script}"'
         # Assume Bash, Zsh, etc.
         # POSIX standard should use dot `. venv/bin/activate` rather than `source`
         return f"source {shlex.quote(str(self.activate_script))}"