Fix quantization of all 0s (#1028)

Author: angeloskath

mlx/ops.cpp

@@ -3274,7 +3274,10 @@ std::tuple<array, array, array> quantize(
       reshape(w, {w.shape(0), w.shape(1) / group_size, group_size}, s);
   array w_max = max(packed_w, /* axis= */ -1, /* keepdims= */ true, s);
   array w_min = min(packed_w, /* axis= */ -1, /* keepdims= */ true, s);
-  array delta = divide(subtract(w_max, w_min, s), array(n_bins, w.dtype()), s);
+  array delta = maximum(
+      divide(subtract(w_max, w_min, s), array(n_bins, w.dtype()), s),
+      array(1e-7, w.dtype()),
+      s);
   array scales = squeeze(delta, -1, s);
   array biases = squeeze(w_min, -1, s);
 

python/tests/test_quantized.py

@@ -18,6 +18,14 @@ def test_quantize_dequantize(self):
                 eps = 1e-6
                 self.assertTrue((errors <= (scales[..., None] + eps)).all())
 
+        # test quantize/dequantize 0s
+        a = mx.zeros((256, 512))
+        for gs in [32, 64, 128]:
+            for b in [2, 4, 8]:
+                w_q, scales, biases = mx.quantize(a, gs, b)
+                a_hat = mx.dequantize(w_q, scales, biases, gs, b)
+                self.assertTrue(mx.all(a_hat == 0))
+
     def test_qmm(self):
         key = mx.random.key(0)
         k1, k2 = mx.random.split(key)