Add support to scale datalist for training

[Nic-Ma]

Is your feature request related to a problem? Please describe.
There is feature request from @dongyang0122 to scale a datalist then use it for training, for example repeat datalist for 4.5 times to make it bigger or scale it to 0.1 smaller for quick test.
We already have the implementation in NVIDIA Clara Train, need to reimplement in MONAI.