fix the bug about loading npy or bin

[iloveai8086]

Thank you for your support of custom datasets.When i was making the dataset with
"python -m pcdet.datasets.custom.custom_dataset create_custom_infos tools/cfgs/dataset_configs/custom_dataset.yaml"
it told me that:
torch.from_numpy(points[:, 0:3]), torch.from_numpy(gt_boxes)
IndexError: too many indices for array: array is 1-dimensional, but 2 were indexed
And i found the function "get_lidar()" in custom_dataset.py was different from kitti_dataset.py.
I think the data should be reshaped

[jihanyang]

We suppose the point clouds are saved in N*C as waymo, since different dataset may have different number of channels.

[iloveai8086]

We suppose the point clouds are saved in N*C as waymo, since different dataset may have different number of channels.

Thank you,I think it would be better for you to add a annotation here.

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@iloveai8086
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