Chronos: a better unscale support is needed

[plusbang]

Currently, we provide TSDataset.unscale_numpy to get unscaled numpy ndarray. We do need a better unscale support rather than ask users to convert to numpy ndarray.

example:

yhat = forecaster.predict(x)
yhat_unscaled = tsdata_test.unscale_numpy(yhat)
y_unscaled = tsdata_test.unscale_numpy(y)
Evaluator.evaluate("mse", y_unscaled, yhat_unscaled)

[TheaperDeng]

maybe add a collate_fn on users' dataloader could be a helpful feature. but we need to define the api first.