test: add comprehensive test suite for LLM components

- Add pytest configuration and fixtures
- Add tests for core modules: decoder, feed_forward, multi_head_attention
- Add tests for positional and token embeddings
- Add tests for GPT model
- Add tests for tokenizers (base and BPE)
- Add basic integration tests

llm/pytest.ini

@@ -0,0 +1,15 @@
+[tool:pytest]
+testpaths = tests
+python_files = test_*.py
+python_classes = Test*
+python_functions = test_*
+addopts = 
+    --verbose
+    --tb=short
+    --strict-markers
+    --strict-config
+    --disable-warnings
+markers =
+    slow: marks tests as slow (deselect with '-m "not slow"')
+    gpu: marks tests that require GPU
+    integration: marks tests as integration tests