[CI][NIXL] Split DPEP tests

[NickLucche]

Separate TP and DPEP end to end tests to better make use of parallel resources (when available), while also enabling DPEP to scale independently in num_gpus for future test scenarios.

[gemini-code-assist]

Code Review

This pull request effectively separates the Tensor Parallelism (TP) and Data Parallelism/Expert Parallelism (DPEP) tests into distinct CI jobs. This is achieved by modifying the .buildkite/test-pipeline.yaml to include a new job for DPEP tests and refactoring the config_sweep_accuracy_test.sh script to select the appropriate test configurations based on the DP_EP environment variable. The changes are well-structured and correctly implement the separation of tests, which should improve CI parallelism and resource utilization. The implementation looks solid.

[gemini-code-assist]

While the logic for selecting test configurations is correct, there is a potential robustness issue in the run_tests function that follows this block (outside the diff). The function passes extra_args to the test script without quotes:

if ! env ${cfg} bash "${SCRIPT}" ${extra_args}; then

This will cause issues if extra_args ever contains arguments with spaces, due to shell word splitting. This is a latent bug that could cause future test failures.

To make the script more robust, I recommend modifying run_tests to handle arguments as an array. For example:

run_tests() {
  local label=$1
  shift
  local extra_args=("$@")

  # ...
    if ! env ${cfg} bash "${SCRIPT}" "${extra_args[@]}"; then
  # ...
}

# ...
run_tests "FLASHINFER backend" --attention-backend FLASHINFER

Since this is outside the diff, I'm adding this comment here for visibility. Addressing this would improve the script's maintainability.