Find a bug while loading matrixcity dataset and fix it

[xinhao-luo]

In scene/dataset_reader.py, line 268:
with open(os.path.join(path, transformsfile)) as json_file:

Actually, the 'transformsfile' here is already a complete dataset path because of line 465:
train_json_path = os.path.join(path, f"transforms_train.json")

So, os.path.join(path, transformsfile) will lead to a bug.

And just keep transformsfile here will be correct.

[TarzanZhao]

Thanks for pointing out!!