Add BotorchTestCase.assertAllClose (#1618)

Summary:
Pull Request resolved: #1618

`BotorchTestCase.assertAllClose` is a thin wrapper around `torch.testing.assert_close`, designed to replace usages of
`self.assertTrue(torch.allclose(...))`.

Using  has several advantages over `torch.allclose`:
* Checks that shapes are equal, not just values
* More configurability and better defaults, such as higher tolerances for single precision, if we choose to use them in the future. for the time being, I've set up this wrapper so that numerical checks remain exactly the same as they used to be.
* More informative test output, showing what was put in and why the test failed:

Old test output:
```AssertionError: False is not true```

New test output:
```
1) AssertionError: Scalars are not close!

Absolute difference: 1.0000034868717194 (up to 0.0001 allowed)
Relative difference: 0.8348668001940709 (up to 1e-05 allowed)
```

Reviewed By: Balandat

Differential Revision: D42402142

fbshipit-source-id: 8d29fde720b24a1c6c9f22797585db5962d01d7b

botorch/utils/testing.py

@@ -9,7 +9,7 @@
 import math
 import warnings
 from collections import OrderedDict
-from typing import Any, List, Optional, Tuple
+from typing import Any, List, Optional, Tuple, Union
 from unittest import TestCase
 
 import torch
@@ -51,6 +51,36 @@ def setUp(self):
             category=UserWarning,
         )
 
+    def assertAllClose(
+        self,
+        input: torch.Tensor,
+        other: Union[torch.Tensor, float],
+        rtol: float = 1e-05,
+        atol: float = 1e-08,
+        equal_nan: bool = False,
+    ) -> None:
+        r"""
+        Calls torch.testing.assert_close, using the signature and default behavior
+        of torch.allclose.
+
+        Example output:
+            AssertionError: Scalars are not close!
+
+            Absolute difference: 1.0000034868717194 (up to 0.0001 allowed)
+            Relative difference: 0.8348668001940709 (up to 1e-05 allowed)
+        """
+        # Why not just use the signature and behavior of `torch.testing.assert_close`?
+        # Because we used `torch.allclose` for testing in the past, and the two don't
+        # behave exactly the same. In particular, `assert_close` requires both `atol`
+        # and `rtol` to be set if either one is.
+        torch.testing.assert_close(
+            input,
+            other,
+            rtol=rtol,
+            atol=atol,
+            equal_nan=equal_nan,
+        )
+
 
 class BaseTestProblemBaseTestCase: