Incorrect spacing is recorded when reading single pydicom dataset object with pydicom

[KumoLiu]

Describe the bug
When reading the dicom series with LoadImage(pydicom backend) in MONAI transforms, the spacing is recorded incorrectly.

updated when reading dicom series:

MONAI/monai/data/image_reader.py

Line 542 in 02e7b79

stack_metadata["spacing"] = np.asarray(spacing)

didn't updated when reading a single pydicom dataset object:

MONAI/monai/data/image_reader.py

Line 634 in 02e7b79

for key in ["00200037", "00200032", "52009229", "52009230"]:

MONAI/monai/data/image_reader.py

Line 664 in 02e7b79

dr, dc = metadata.get("spacing", (1.0, 1.0))[:2]

To Reproduce
Steps to reproduce the behavior:

1. Install the MONAI dev branch.
2. Download the attachment dcm file whose md5 is f511acb750076c56ad2d9647dcfe521d, put it in a folder.
3. Run

from monai.transforms import LoadImage
loader = LoadImage(reader='pydicomreader')
pydicom_result = loader('./test_data')
print(pydicom_result.affine)

loader = LoadImage(reader='itkreader')
itk_result = loader('./test_data')
print(itk_result.affine)

Expected behavior
pydicom can record spacing correctly when reading a single pydicom dataset object