FastMRI reader outputs Tensor which is not supported by LoadImage Transform

[Can-Zhao]

Describe the bug
FastMRI reader outputs Tensor during inference, which is not supported by LoadImage Transform

To Reproduce
Steps to reproduce the behavior:

1. Go to '...'
2. Install '....'
3. Run commands '....'

Expected behavior
switch_endianness function reports the bug, due to unsupport of torch.Tensor output by FastMRI reader.

We should either let FastMRI reader to output numpy array, or let switch_endianness support Tensor.

Screenshots
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Environment

Ensuring you use the relevant python executable, please paste the output of:

python -c 'import monai; monai.config.print_debug_info()'

Additional context
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