Add pytest coverage for liger-kernel (#876)

## Summary
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this proposed code change. --->

This PR adds [pytest-cov](https://pypi.org/project/pytest-cov/) which
helps measure test coverage and can help improve overall test coverage/
diagnose test which are slow or failing. Also, I've added
[pytest-asyncio](https://pypi.org/project/pytest-asyncio/) which was
being used in pyproject.toml, but was not added as a dev dependency.

Could you please review?

cc: @lancerts , @shimizust 
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## Details
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should be aware of?
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## Testing Done
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tested. --->

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- Hardware Type: <BLANK>
- [ ] run `make test` to ensure correctness
- [ ] run `make checkstyle` to ensure code style
- [ ] run `make test-convergence` to ensure convergence

Author: ParagEkbote

.gitignore

@@ -23,4 +23,5 @@ uv.lock
 
 # Benchmark images
 benchmark/visualizations
-.vscode/
+.vscode/
+.coverage

Makefile

@@ -5,7 +5,16 @@ all: checkstyle test test-convergence
 
 # Command to run pytest for correctness tests
 test:
-	python -m pytest --disable-warnings test/ --ignore=test/convergence
+	python -m pytest --disable-warnings \
+		-n auto \
+		--dist=load \
+		--cov=src/liger_kernel \
+		--cov-report=term-missing \
+		--ignore=test/convergence \
+		test/
+	coverage combine
+	coverage report -m
+	coverage html
 
 # Command to run ruff for linting and formatting code
 checkstyle:

pyproject.toml

@@ -24,6 +24,36 @@ pythonpath = ["src", "."]
 asyncio_mode = "auto"
 log_cli = true
 log_cli_level = "INFO"
+addopts = [
+    "-n", "auto",
+    "--dist=load",                    # use "load" to distribute tests and let pytest-cov combine coverage
+    "--cov=src/liger_kernel",
+    "--cov-report=term-missing",
+    "--cov-report=html",
+    "--cov-config=pyproject.toml",
+    "--durations=0"
+]
+python_files = "test_*.py"
+testpaths = ["test/"]
+
+[tool.coverage.run]
+branch = true
+parallel = true
+source = ["src/liger_kernel"]
+# xdist uses subprocesses; "multiprocessing" is a safe concurrency choice
+concurrency = ["multiprocessing"]
+
+[tool.coverage.paths]
+liger_kernel = [
+  "src/liger_kernel",
+  "*/site-packages/liger_kernel"
+]
+
+[tool.coverage.report]
+omit = ["test/*"]
+show_missing = true
+skip_covered = false
+
 
 [tool.ruff]
 line-length = 120

setup.py

@@ -38,6 +38,8 @@ def get_optional_dependencies():
             "isort>=5.13.2",
             "pytest>=7.1.2",
             "pytest-xdist",
+            "pytest-cov",
+            "pytest-asyncio",
             "pytest-rerunfailures",
             "datasets>=2.19.2",
             "seaborn",