Added better default compute_dtype handling for Linear4bit layers.

bitsandbytes/nn/modules.py

@@ -4,6 +4,7 @@
 # LICENSE file in the root directory of this source tree.
 from typing import Optional, TypeVar, Union, overload
 
+import warnings
 import torch
 import torch.nn.functional as F
 from torch import Tensor, device, dtype, nn
@@ -205,6 +206,28 @@ class Linear4bit(nn.Linear):
         super().__init__(input_features, output_features, bias, device)
         self.weight = Params4bit(self.weight.data, requires_grad=False, compress_statistics=compress_statistics, quant_type=quant_type)
         self.compute_dtype = compute_dtype
+        self.compute_type_is_set = False
+
+    def set_compute_type(self, x):
+        if x.dtype in [torch.float32, torch.bfloat16]:
+            # the input is in a dtype that is safe to compute in, we switch
+            # to this type for speed and stability
+            self.compute_dtype = x.dtype
+        elif x.dtype == torch.float16:
+            # we take the compoute dtype passed into the layer
+            if self.compute_dtype == torch.float32 and (x.numel() == x.shape[-1]):
+                # single batch inference with input torch.float16 and compute_dtype float32 -> slow inference when it could be fast
+                # warn the user about this
+                warnings.warn(f'Input type into Linear4bit is torch.float16, but bnb_4bit_compute_type=torch.float32 (default). This will lead to slow inference.')
+                warnings.filterwarnings('ignore', message='.*inference.')
+            if self.compute_dtype == torch.float32 and (x.numel() != x.shape[-1]):
+                warnings.warn(f'Input type into Linear4bit is torch.float16, but bnb_4bit_compute_type=torch.float32 (default). This will lead to slow inference or training speed.')
+                warnings.filterwarnings('ignore', message='.*inference or training')
+
+
+
+
+
 
     def forward(self, x: torch.Tensor):
         # weights are cast automatically as Int8Params, but the bias has to be cast manually
@@ -213,6 +236,10 @@ class Linear4bit(nn.Linear):
 
         if getattr(self.weight, 'quant_state', None) is None:
             print('FP4 quantization state not initialized. Please call .cuda() or .to(device) on the LinearFP4 layer first.')
+        if not self.compute_type_is_set:
+            self.set_compute_type(x)
+            self.compute_type_is_set = True
+
         inp_dtype = x.dtype
         if self.compute_dtype is not None:
             x = x.to(self.compute_dtype)

tests/test_modules.py

@@ -516,7 +516,10 @@ modules.append(bnb.nn.LinearFP4)
 modules.append(bnb.nn.LinearNF4)
 modules.append(lambda d1, d2: bnb.nn.LinearFP4(d1, d2, compress_statistics=True))
 modules.append(lambda d1, d2: bnb.nn.LinearNF4(d1, d2, compress_statistics=True))
-names = ['Int8Lt', '4bit', 'FP4', 'NF4', 'FP4+C', 'NF4+C']
+modules.append(lambda d1, d2: bnb.nn.LinearFP4(d1, d2, compute_dtype=torch.float32))
+modules.append(lambda d1, d2: bnb.nn.LinearFP4(d1, d2, compute_dtype=torch.float16))
+modules.append(lambda d1, d2: bnb.nn.LinearFP4(d1, d2, compute_dtype=torch.bfloat16))
+names = ['Int8Lt', '4bit', 'FP4', 'NF4', 'FP4+C', 'NF4+C', 'NF4+fp32', 'NF4+fp16', 'NF4+bf16']
 @pytest.mark.skipif(not torch.cuda.is_available(), reason="this test requires a GPU")
 @pytest.mark.parametrize("module", modules, ids=names)
 def test_kbit_backprop(module):
@@ -563,10 +566,10 @@ def test_kbit_backprop(module):
         relerrs2.append(relerr2.mean().item())
 
         if isinstance(module, bnb.nn.Linear8bitLt):
-            torch.testing.assert_close(grad1, grad2, atol=0.008, rtol=0.05)
+            assert_all_approx_close(grad1, grad2, atol=0.008, rtol=0.05, count=1)
             torch.testing.assert_close(bgrad1, bgrad2, atol=0.008, rtol=0.05)
         else:
-            torch.testing.assert_close(grad1, grad2, atol=0.015, rtol=0.05)
+            assert_all_approx_close(grad1, grad2, atol=0.015, rtol=0.05, count=1)
             torch.testing.assert_close(bgrad1, bgrad2, atol=0.02, rtol=0.05)
         ref.zero_grad()
         kbit.zero_grad()
@@ -608,9 +611,33 @@ def test_fp8linear():
     assert graderr < 0.00002
     assert bgraderr < 0.00002
 
-
-
-
+def test_4bit_warnings():
+    dim1 = 64
+
+    with pytest.warns(UserWarning, match=r'inference or training'):
+        net = nn.Sequential(*[bnb.nn.Linear4bit(dim1, dim1, compute_dtype=torch.float32) for i in range(10)])
+        net = net.cuda()
+        inp = torch.rand(10, dim1).cuda().half()
+        net(inp)
+    with pytest.warns(UserWarning, match=r'inference.'):
+        net = nn.Sequential(*[bnb.nn.Linear4bit(dim1, dim1, compute_dtype=torch.float32) for i in range(10)])
+        net = net.cuda()
+        inp = torch.rand(1, dim1).cuda().half()
+        net(inp)
+
+    with pytest.warns(UserWarning) as record:
+
+        net = nn.Sequential(*[bnb.nn.Linear4bit(dim1, dim1, compute_dtype=torch.float32) for i in range(10)])
+        net = net.cuda()
+        inp = torch.rand(10, dim1).cuda().half()
+        net(inp)
+
+        net = nn.Sequential(*[bnb.nn.Linear4bit(dim1, dim1, compute_dtype=torch.float32) for i in range(10)])
+        net = net.cuda()
+        inp = torch.rand(1, dim1).cuda().half()
+        net(inp)
+
+    assert len(record) == 2