[Fix] Convert rt_mat to torch.Tensor in Coord transform for compatibility

[gopi-erabati]

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Motivation

Convert rt_mat in 'coord_3d_mode.py' from np.ndarray to torch.Tensor. Closes #707

Modification

The rt_mat in 'coord_3d_mode.py' takes np.ndarray and torch.tensor types, but the conversion from np.ndarray to torch.Tensor is missing as further code assumes that rt_mat is a torch.Tensor type. So, conversion of rt_mat from np.ndarray to torch.tensor type is added!

BC-breaking (Optional)

Does the modification introduce changes that break the back-compatibility of the downstream repos?
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)

If this PR introduces a new feature, it is better to list some use cases here, and update the documentation.

Checklist

1. Pre-commit or other linting tools are used to fix the potential lint issues.
2. The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
3. If the modification has potential influence on downstream projects, this PR should be tested with downstream projects.
4. The documentation has been modified accordingly, like docstring or example tutorials.

[CLAassistant]

All committers have signed the CLA.

[Wuziyi616]

Only need to fix that small comment and wait for CI to pass.

[codecov]

Codecov Report

Merging #709 (7c68a15) into master (a1e0453) will decrease coverage by 0.00%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #709      +/-   ##
==========================================
- Coverage   48.93%   48.92%   -0.01%     
==========================================
  Files         208      208              
  Lines       15861    15863       +2     
  Branches     2543     2544       +1     
==========================================
  Hits         7761     7761              
- Misses       7606     7607       +1     
- Partials      494      495       +1     

Flag	Coverage Δ
unittests	48.92% <0.00%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
mmdet3d/core/bbox/structures/coord_3d_mode.py	60.99% <0.00%> (-0.88%)	⬇️

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