Fix Windows cuda compilation error on floor()

[LudoBar]

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Motivation

Compilation errors in CUDA kernels on floor() calls. Type mismatch, floorf must be used for float.

Modification

Replace floor() by floorf().

Environment

Windows 10
Visual Studio Community 2019 (v16.9.4)
MSVC++ 14.28
CUDA 11.1

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.

Checklist

Before PR:

• I have read and followed the workflow indicated in the CONTRIBUTING.md to create this PR.
• Pre-commit or linting tools indicated in CONTRIBUTING.md are used to fix the potential lint issues.
• Bug fixes are covered by unit tests, the case that causes the bug should be added in the unit tests.
• New functionalities are covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• The documentation has been modified accordingly, including docstring or example tutorials.

After PR:

• If the modification has potential influence on downstream or other related projects, this PR should be tested with some of those projects, like MMDet or MMCls.
• CLA has been signed and all committers have signed the CLA in this PR.

[CLAassistant]

All committers have signed the CLA.

[zhouzaida]

Related PR: #891 #1031

[grimoire]

LGTM

[OpenMMLab-Assistant003]

Hi @LudoBar！First of all, we want to express our gratitude for your significant PR in the MMCV project. Your contribution is highly appreciated, and we are grateful for your efforts in helping improve this open-source project during your personal time. We believe that many developers will benefit from your PR.

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Thank you again for your contribution❤ @LudoBar