[CI/Docs] Improve aarch64/DGX Spark support for dev setup (#38057)

Signed-off-by: Ben Browning <bbrownin@redhat.com>
Signed-off-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>
Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>
Co-authored-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>

AGENTS.md

@@ -39,6 +39,8 @@ If work is duplicate/trivial busywork, **do not proceed**. Return a short explan
 
 ## 2. Development Workflow
 
+- **Never use system `python3` or bare `pip`/`pip install`.** All Python commands must go through `uv` and `.venv/bin/python`.
+
 ### Environment setup
 
 ```bash
@@ -58,33 +60,33 @@ pre-commit install
 
 ```bash
 # If you are only making Python changes:
-VLLM_USE_PRECOMPILED=1 uv pip install -e .
+VLLM_USE_PRECOMPILED=1 uv pip install -e . --torch-backend=auto
 
 # If you are also making C/C++ changes:
-uv pip install -e .
+uv pip install -e . --torch-backend=auto
 ```
 
 ### Running tests
 
-Tests require extra dependencies.
-All versions for test dependencies should be read from `requirements/test.txt`
+> Requires [Environment setup](#environment-setup) and [Installing dependencies](#installing-dependencies).
 
 ```bash
-# Install bare minimum test dependencies:
-uv pip install pytest pytest-asyncio tblib
-
-# Install additional test dependencies as needed, or install them all as follows:
+# Install test dependencies.
+# requirements/test.txt is pinned to x86_64; on other platforms, use the
+# unpinned source file instead:
+uv pip install -r requirements/test.in    # resolves for current platform
+# Or on x86_64:
 uv pip install -r requirements/test.txt
 
-# Run specific test from specific test file
-pytest tests/path/to/test.py -v -s -k test_name
-
-# Run all tests in directory
-pytest tests/path/to/dir -v -s
+# Run a specific test file (use .venv/bin/python directly;
+# `source activate` does not persist in non-interactive shells):
+.venv/bin/python -m pytest tests/path/to/test_file.py -v
 ```
 
 ### Running linters
 
+> Requires [Environment setup](#environment-setup).
+
 ```bash
 # Run all pre-commit hooks on staged files:
 pre-commit run

requirements/test.in

@@ -61,7 +61,7 @@ runai-model-streamer[s3,gcs,azure]==0.15.7
 fastsafetensors>=0.2.2 # 0.2.2 contains important fixes for multi-GPU mem usage
 instanttensor>=0.1.5
 pydantic>=2.12 # 2.11 leads to error on python 3.13
-decord==0.6.0
+decord==0.6.0; platform_machine == "x86_64"
 terratorch >= 1.2.2 # Required for Prithvi tests
 imagehash # Required for Prithvi tests
 segmentation-models-pytorch > 0.4.0 # Required for Prithvi tests