fix grad_output shape error

[103yiran]

Motivation

fix grad_output shape in modulated_deform_conv_backward_fallthrough.

Modification

$N \times G \times \frac{C_o}{G} \times H_o \times W_o \to N \times C_o \times H_o \times W_o$

BC-breaking (Optional)

Does the modification introduce changes that break the backward-compatibility of the downstream repositories?
If so, please describe how it breaks the compatibility and how the downstream projects should modify their code to keep compatibility with this PR.

Use cases (Optional)

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Checklist

Before PR:

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• Bug fixes are covered by unit tests, the case that causes the bug should be added in the unit tests.
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