feat: added few comments and renamed symbol for more clearility

gpt2.py

@@ -35,8 +35,9 @@ def attention(q, k, v, mask):  # [n_q, d_k], [n_k, d_k], [n_k, d_v], [n_q, n_k]
     return softmax(q @ k.T / np.sqrt(q.shape[-1]) + mask) @ v
 
 
-def mha(x, c_attn, c_proj, n_head, kvcache):  # [n_seq, n_embd] -> [n_seq, n_embd]
+def mha(x, c_attn, c_proj, n_head, kvcache=None):  # [n_seq, n_embd] -> [n_seq, n_embd]
     # qkv projection
+    # when we pass kvcache, n_seq = 1. so we will compute new_q, new_k and new_v
     x = linear(x, **c_attn)  # [n_seq, n_embd] -> [n_seq, 3*n_embd]
 
     # split into qkv
@@ -44,11 +45,11 @@ def mha(x, c_attn, c_proj, n_head, kvcache):  # [n_seq, n_embd] -> [n_seq, n_emb
 
     if kvcache:
         # qkv
-        q, k, v = qkv
+        new_q, new_k, new_v = qkv  # new_q, new_k, new_v = [1, n_embd]
         old_k, old_v = kvcache
-        k = np.vstack([old_k, k])
-        v = np.vstack([old_v, v])
-        qkv = [q, k, v]
+        k = np.vstack([old_k, new_k]) # k = [n_seq, n_embd], where n_seq = prev_n_seq + 1
+        v = np.vstack([old_v, new_v]) # v = [n_seq, n_embd], where n_seq = prev_n_seq + 1
+        qkv = [new_q, k, v]
 
     current_cache = [qkv[1], qkv[2]]
 
@@ -57,8 +58,10 @@ def mha(x, c_attn, c_proj, n_head, kvcache):  # [n_seq, n_embd] -> [n_seq, n_emb
 
     # causal mask to hide future inputs from being attended to
     if kvcache: 
+        # when we pass kvcache, we are passing single token as input which need to attend to all previous tokens, so we create mask with all 0s
         causal_mask = np.zeros((1, k.shape[0]))
     else:
+        # create triangular causal mask
         causal_mask = (1 - np.tri(x.shape[0])) * -1e10  # [n_seq, n_seq]
 
     # perform attention over each head
@@ -74,7 +77,7 @@ def mha(x, c_attn, c_proj, n_head, kvcache):  # [n_seq, n_embd] -> [n_seq, n_emb
     return x, current_cache
 
 
-def transformer_block(x, mlp, attn, ln_1, ln_2, n_head, kvcache):  # [n_seq, n_embd] -> [n_seq, n_embd]
+def transformer_block(x, mlp, attn, ln_1, ln_2, n_head, kvcache=None):  # [n_seq, n_embd] -> [n_seq, n_embd]
     # multi-head causal self attention
     attn_out, kvcache_updated = mha(layer_norm(x, **ln_1), **attn, n_head=n_head, kvcache=kvcache)
     x = x + attn_out  # [n_seq, n_embd] -> [n_seq, n_embd]
@@ -85,7 +88,7 @@ def transformer_block(x, mlp, attn, ln_1, ln_2, n_head, kvcache):  # [n_seq, n_e
     return x, kvcache_updated
 
 
-def gpt2(inputs, wte, wpe, blocks, ln_f, n_head, kvcache):  # [n_seq] -> [n_seq, n_vocab]
+def gpt2(inputs, wte, wpe, blocks, ln_f, n_head, kvcache = None):  # [n_seq] -> [n_seq, n_vocab]
     if not kvcache:
         kvcache = [None]*len(blocks)
         wpe_out = wpe[range(len(inputs))]