Fix max_width computation in _tensor_str._Formatter (#126859)

Previous version of `torch._tensor_str._Formatter` was not using `PRINT_OPTS.sci_mode` for the `max_width` computation but was using it for the formatting of values leading to a weird discrepancy.

Now, the code first checks if it should be in sci_mode, then compute `max_width`

Here is an example to test the behavior:
```python
A = torch.tensor([10, 1e-1, 1e-2])
B = torch.tensor([10, 1e-1, 1e-1])

print("================= Default =================")
print(A, f"Formatter max_width: {torch._tensor_str._Formatter(A).max_width}")
print(B, f"Formatter max_width: {torch._tensor_str._Formatter(B).max_width}")

print("================= sci_mode=False =================")
with torch._tensor_str.printoptions(sci_mode=False):
    print(A, f"Formatter max_width: {torch._tensor_str._Formatter(A).max_width}")
    print(B, f"Formatter max_width: {torch._tensor_str._Formatter(B).max_width}")

print("================= sci_mode=True =================")
with torch._tensor_str.printoptions(sci_mode=True):
    print(A, f"Formatter max_width: {torch._tensor_str._Formatter(A).max_width}")
    print(B, f"Formatter max_width: {torch._tensor_str._Formatter(B).max_width}")
```

In the current version this prints:
```
================= Default =================
tensor([1.0000e+01, 1.0000e-01, 1.0000e-02]) Formatter max_width: 10
tensor([10.0000,  0.1000,  0.1000]) Formatter max_width: 7
================= sci_mode=False =================
tensor([   10.0000,     0.1000,     0.0100]) Formatter max_width: 10
tensor([10.0000,  0.1000,  0.1000]) Formatter max_width: 7
================= sci_mode=True =================
tensor([1.0000e+01, 1.0000e-01, 1.0000e-02]) Formatter max_width: 10
tensor([1.0000e+01, 1.0000e-01, 1.0000e-01]) Formatter max_width: 7
```

On can see that in `sci_mode=False`, the values of A are prefixed with unneeded 0 and does not have the same `max_width` as B (It keeps the `max_width` from `sci_mode = None`)

Also in `sci_mode = True`, for B, the `max_width` is 7 but each value takes 10 chars... (But it is fine as the code that uses `max_width` do not rely much on it, but still, this is missleading)

After this commit, this will print
```
================= Default =================
tensor([1.0000e+01, 1.0000e-01, 1.0000e-02]) Formatter max_width: 10
tensor([10.0000,  0.1000,  0.1000]) Formatter max_width: 7
================= sci_mode=False =================
tensor([10.0000,  0.1000,  0.0100]) Formatter max_width: 7
tensor([10.0000,  0.1000,  0.1000]) Formatter max_width: 7
================= sci_mode=True =================
tensor([1.0000e+01, 1.0000e-01, 1.0000e-02]) Formatter max_width: 10
tensor([1.0000e+01, 1.0000e-01, 1.0000e-01]) Formatter max_width: 10
```

This also allows to align A with B for `sci_mode=False`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/126859
Approved by: https://github.com/malfet

test/test_torch.py

@@ -8337,7 +8337,7 @@ class TestTorch(TestCase):
         self.assertExpectedInline(str(x), '''tensor([1.0000e+02, 1.0000e-02])''')
         torch.set_printoptions(sci_mode=False)
         self.assertEqual(x.__repr__(), str(x))
-        self.assertExpectedInline(str(x), '''tensor([  100.0000,     0.0100])''')
+        self.assertExpectedInline(str(x), '''tensor([100.0000,   0.0100])''')
         torch.set_printoptions(sci_mode=None)  # reset to the default value
 
         # test no leading space if all elements positive