Simplify pip install step of install_requirements.sh

Summary:
It's not clear why we were using --force-reinstall; if the requested
versions are different from what's currently installed, pip will handle
the reinstallation.

And calling pip multiple times with --force-reinstall meant that we were
uninstalling and reinstalling the same packages over and over.

Now we only call `pip install` once for requirements, and we do not
force reinstall. On my machine, without enabling pybindings, a no-op run
of this script (i.e. when all packages are up-to-date) takes 7 seconds,
including the `pip install .`.

Test Plan:
```
./install_requirements.sh
```

Reviewers:

Subscribers:

Tasks:

Tags:

Author: dbort

install_requirements.sh

@@ -41,35 +41,62 @@ for arg in "$@"; do
   esac
 done
 
-# Install pytorch dependencies
 #
-# Note:
-# When getting a new version of the executorch repo (via clone, fetch, or pull),
-# you may need to re-install a new version for all pytorch dependencies to run the
-# models in executorch/examples/models.
-# The version in this file will be the correct version for the
-# corresponsing version of the repo.
+# Install pip packages used by code in the ExecuTorch repo.
+#
+
+# Since ExecuTorch often uses main-branch features of pytorch, only the nightly
+# pip versions will have the required features. The NIGHTLY_VERSION value should
+# agree with the third-party/pytorch pinned submodule commit.
+#
+# NOTE: If a newly-fetched version of the executorch repo changes the value of
+# NIGHTLY_VERSION, you should re-run this script to install the necessary
+# package versions.
 NIGHTLY_VERSION=dev20240324
 
-TORCH_VERSION=2.4.0.${NIGHTLY_VERSION}
-pip install --force-reinstall --pre torch=="${TORCH_VERSION}" -i https://download.pytorch.org/whl/nightly/cpu
+# The pip repository that hosts nightly torch packages.
+TORCH_NIGHTLY_URL="https://download.pytorch.org/whl/nightly/cpu"
 
-TORCH_VISION_VERSION=0.19.0.${NIGHTLY_VERSION}
-pip install --force-reinstall --pre torchvision=="${TORCH_VISION_VERSION}" -i https://download.pytorch.org/whl/nightly/cpu
+# pip packages needed by exir.
+EXIR_REQUIREMENTS=(
+  torch==2.4.0.${NIGHTLY_VERSION}
+  torchaudio==2.2.0.${NIGHTLY_VERSION} # For testing.
+)
 
-TORCH_AUDIO_VERSION=2.2.0.${NIGHTLY_VERSION}
-pip install --force-reinstall --pre torchaudio=="${TORCH_AUDIO_VERSION}" -i https://download.pytorch.org/whl/nightly/cpu
+# pip packages needed for development.
+DEVEL_REQUIREMENTS=(
+  setuptools  # For building the pip package.
+  tomli  # Imported by extract_sources.py when using python < 3.11.
+  wheel  # For building the pip package archive.
+  zstd  # Imported by resolve_buck.py.
+)
 
-TIMM_VERSION=0.6.13
-pip install --pre timm==${TIMM_VERSION}
+# pip packages needed to run examples.
+# TODO(dbort): Make each example publish its own requirements.txt
+EXAMPLES_REQUIREMENTS=(
+  timm==0.6.13
+  torchaudio==2.2.0.${NIGHTLY_VERSION}
+  torchsr==1.0.4
+  transformers==4.38.2
+)
 
-TRANSFORMERS_VERSION=4.38.2
-pip install --force-reinstall --pre transformers==${TRANSFORMERS_VERSION}
+# Assemble the list of requirements to actually install.
+# TODO(dbort): Add options for reducing the number of requirements.
+REQUIREMENTS_TO_INSTALL=(
+  "${EXIR_REQUIREMENTS[@]}"
+  "${DEVEL_REQUIREMENTS[@]}"
+  "${EXAMPLES_REQUIREMENTS[@]}"
+)
 
-TORCHSR_VERSION=1.0.4
-pip install --pre torchsr==${TORCHSR_VERSION}
+# Install the requirements. `--extra-index-url` tells pip to look for package
+# versions on the provided URL if they aren't available on the default URL.
+pip install --extra-index-url "${TORCH_NIGHTLY_URL}" \
+    "${REQUIREMENTS_TO_INSTALL[@]}"
+
+#
+# Install executorch pip package.
+#
 
-# Install ExecuTorch after dependencies are installed.
 EXECUTORCH_BUILD_PYBIND="$EXECUTORCH_BUILD_PYBIND" \
 CMAKE_ARGS="$CMAKE_ARGS" \
 CMAKE_BUILD_PARALLEL_LEVEL=9 \