also add a test that failed before the fix and passes now with the fix for kv cache resize

tests/test_engine.py

@@ -0,0 +1,66 @@
+"""
+Test Engine class. Example run:
+
+python -m pytest tests/test_engine.py -v
+"""
+
+import torch
+from nanochat.engine import KVCache
+
+def test_kv_cache_resize():
+    """
+    The KV cache was not resized correctly, more information here:
+    https://github.com/karpathy/nanochat/pull/186
+    This test reproduces the issue and will be merged alongside the fix.
+    """
+
+    batch_size = 2
+    num_heads = 3
+    seq_len = 4
+    head_dim = 5
+    num_layers = 6
+
+    kv_cache = KVCache(
+        batch_size=batch_size,
+        num_heads=num_heads,
+        seq_len=seq_len,
+        head_dim=head_dim,
+        num_layers=num_layers
+    )
+
+    # Insert a single token with a distinct fill value to all layers
+    def insert_token(token_idx):
+        for layer_idx in range(num_layers):
+            k = torch.full((batch_size, num_heads, 1, head_dim), fill_value=float(token_idx), dtype=torch.float32)
+            v = torch.full((batch_size, num_heads, 1, head_dim), fill_value=float(token_idx * 100), dtype=torch.float32)
+            kv_cache.insert_kv(layer_idx, k, v)
+
+    # Insert 4 tokens (fills the initial seq_len=4)
+    for i in range(4):
+        insert_token(i)
+
+    # Record the original state of the cache
+    original_cache = kv_cache.kv_cache.clone()
+    original_seq_len = original_cache.shape[4]
+
+    # Insert the 5th token, which will trigger a resize
+    insert_token(4)
+    # Verify that the cache actually resized
+    new_seq_len = kv_cache.kv_cache.shape[4]
+    assert new_seq_len > original_seq_len, f"Cache did not resize: original seq_len={original_seq_len}, new seq_len={new_seq_len}"
+
+    # Verify that the original 4 tokens are still intact after resize
+    for layer_idx in range(num_layers):
+        for token_idx in range(4):
+            # Check that resized cache matches expected values
+            expected_k = float(token_idx)
+            expected_v = float(token_idx * 100)
+            actual_k = kv_cache.kv_cache[layer_idx, 0, :, :, token_idx, :]
+            actual_v = kv_cache.kv_cache[layer_idx, 1, :, :, token_idx, :]
+            assert (actual_k == expected_k).all(), f"Layer {layer_idx}, token {token_idx}: key corrupted, expected {expected_k}"
+            assert (actual_v == expected_v).all(), f"Layer {layer_idx}, token {token_idx}: value corrupted, expected {expected_v}"
+            # And that the original cache matches resized cache
+            original_k = original_cache[layer_idx, 0, :, :, token_idx, :]
+            original_v = original_cache[layer_idx, 1, :, :, token_idx, :]
+            assert (actual_k == original_k).all(), f"Layer {layer_idx}, token {token_idx}: key doesn't match original"
+            assert (actual_v == original_v).all(), f"Layer {layer_idx}, token {token_idx}: value doesn't match original"