[hotfix] IsContiguous failed when tensor's size on first dimension is 1.

[yzh119]

Description

See #2118 .

When tensor's size on first dimension is 1, the stride on first dimension does not strictly equals the production of sizes on following dimensions:

>>> torch.randn((1, 5, 3))[:, :3, :].contiguous().stride()  # (15, 3, 1), however, we expect (9, 3, 1)

Checklist

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[BarclayII]

Not necessarily the first dimension. The tensor is considered contiguous if the first few dimensions are all 1.

https://github.com/pytorch/pytorch/blob/60ad7e9c043b566db44bf648b71496a99fbc22d6/c10/core/TensorImpl.cpp#L84-L100

[yzh119]

I see