lambda transform should be optionally randomisable

[wyli]

Is your feature request related to a problem? Please describe.
the lambda transform may have some random factors (such as using np.random), in that case it shouldn't be cached in a cache dataset. Ideally the lambda transform should support dynamic inheritance of the randomisable interface

[rijobro]

Should there be a RandLambda transform such that the CacheDataset knows whether it's a random transformation or not?

[Nic-Ma]

RandLambda sounds like a good solution, I will add it soon.
Thanks.