Assertion fails when torch.mean is called with dtype argument.

## 🐞Describing the bug
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Assertion fails when torch.mean is called with dtype argument.

https://github.com/apple/coremltools/blob/0b713d23ba86fa6cdb23e8b24e9e65e0f8c35807/coremltools/converters/mil/frontend/torch/ops.py#L1615

## Stack Trace
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## To Reproduce
- Please add a minimal code example that can reproduce the error when running it.
```
# Paste Python code snippet here, complete with any required import statements.
```
- If the model conversion succeeds, but there is a numerical mismatch in predictions, please include the code used for comparisons.

## System environment (please complete the following information):
- coremltools version: 7.1
- OS (e.g. MacOS version or Linux type): macOS Sonoma 14.3
- Any other relevant version information (e.g. PyTorch or TensorFlow version):

## Additional context
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