Fix padding for int contexts (#227)

*Issue #, if available:* On Linux, the final call to `.to` creates
trouble when input tensors are integer. For example:

```
>>> a = torch.tensor([1])
>>> b = torch.stack([torch.full((1,), torch.nan), a])
>>> b
tensor([[nan],
        [1.]])
>>> b.to(a)
tensor([[-9223372036854775808],
        [                   1]])
```


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choice.

src/chronos/utils.py

@@ -17,4 +17,4 @@ def left_pad_and_stack_1D(tensors: List[torch.Tensor]) -> torch.Tensor:
             size=(max_len - len(c),), fill_value=torch.nan, device=c.device
         )
         padded.append(torch.concat((padding, c), dim=-1))
-    return torch.stack(padded).to(tensors[0])
+    return torch.stack(padded)

test/__init__.py

@@ -1,2 +1,2 @@
 # Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
-# SPDX-License-Identifier: Apache-2.0
+# SPDX-License-Identifier: Apache-2.0

test/test_utils.py

@@ -0,0 +1,29 @@
+# Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+# SPDX-License-Identifier: Apache-2.0
+
+import pytest
+import torch
+
+from chronos.utils import left_pad_and_stack_1D
+
+
+@pytest.mark.parametrize(
+    "tensors",
+    [
+        [
+            torch.tensor([2.0, 3.0], dtype=dtype),
+            torch.tensor([4.0, 5.0, 6.0], dtype=dtype),
+            torch.tensor([7.0, 8.0, 9.0, 10.0], dtype=dtype),
+        ]
+        for dtype in [torch.int, torch.float16, torch.float32]
+    ],
+)
+def test_pad_and_stack(tensors: list):
+    stacked_and_padded = left_pad_and_stack_1D(tensors)
+
+    assert stacked_and_padded.dtype == torch.float32
+    assert stacked_and_padded.shape == (len(tensors), max(len(t) for t in tensors))
+
+    ref = torch.concat(tensors).to(dtype=stacked_and_padded.dtype)
+
+    assert torch.sum(torch.nan_to_num(stacked_and_padded, nan=0)) == torch.sum(ref)

test/util.py

@@ -10,4 +10,4 @@ def validate_tensor(
     assert a.shape == shape
 
     if dtype is not None:
-        assert a.dtype == dtype
+        assert a.dtype == dtype