An initial quantized matmul implementation (#205)

* Add quantized matvec
* Add quantized matrix matrix with 2nd matrix transposed
* Add quantized matmul tests
* Add a slow cpu quantized matmul
* Add a slightly faster vectorized cpu version

benchmarks/python/comparative/bench_mlx.py

@@ -23,6 +23,16 @@ def none_or_list(x):
         return [int(xi) for xi in x.split(",")]
 
 
+def dtype_from_str(x):
+    if x == "":
+        return mx.float32
+    else:
+        dt = getattr(mx, x)
+        if not isinstance(dt, mx.Dtype):
+            raise ValueError(f"{x} is not an mlx dtype")
+        return dt
+
+
 def bench(f, *args):
     for i in range(10):
         f(*args)
@@ -49,6 +59,15 @@ def matmul(x, y):
     mx.eval(ys)
 
 
+def quant_matmul(x, w, s, b):
+    groups = x.shape[-1] // s.shape[-1]
+    width = 32 // (x.shape[-1] // w.shape[0])
+    ys = []
+    for i in range(10):
+        ys.append(mx.quantized_matmul(x, w, s, b, groups=groups, width=width))
+    mx.eval(ys)
+
+
 def conv1d(x, y):
     ys = []
     for i in range(10):
@@ -296,9 +315,7 @@ if __name__ == "__main__":
     parser.add_argument(
         "--fused", action="store_true", help="Use fused functions where possible"
     )
-    parser.add_argument(
-        "--dtype", choices=["float32", "float16", "bfloat16"], default="float32"
-    )
+    parser.add_argument("--dtype", type=dtype_from_str, default=[], action="append")
 
     args = parser.parse_args()
 
@@ -315,11 +332,15 @@ if __name__ == "__main__":
         mx.set_default_device(mx.cpu)
     else:
         mx.set_default_device(mx.gpu)
-    dtype = dict(float32=mx.float32, float16=mx.float16, bfloat16=mx.bfloat16)[
-        args.dtype
-    ]
+
+    types = args.dtype
+    if not types:
+        types = [mx.float32]
+    if len(types) < len(args.size):
+        types = types + [types[0]] * (len(args.size) - len(types))
+
     xs = []
-    for size in args.size:
+    for size, dtype in zip(args.size, types):
         xs.append(mx.random.normal(size).astype(dtype))
     for i, t in enumerate(args.transpose):
         if t is None:
@@ -335,6 +356,9 @@ if __name__ == "__main__":
     elif args.benchmark == "matmul":
         print(bench(matmul, *xs))
 
+    elif args.benchmark == "quant_matmul":
+        print(bench(quant_matmul, *xs))
+
     elif args.benchmark == "linear":
         print(bench(linear, *xs))