hot-fix for the bug of remaining future values

[ktaaaki]

This is hot fix of IterativeLoss.

Issue: a bug of IterativeLoss

1. x_dict.update({value: x_dict[key]}) at line:156 register key-variable's value as value-variable's value. And key-variable remains.
2. At next step, key-variable are sometimes overwritten but sometimes not. For example, AddLoss copies an input dictionary, and overwrite old one to updated one.
3. the key-variable is not updated. So the effect of reccurence is ignored.

Solution

• hotfix: delete future variable by replace_dict_keys.
• raise exception when some variables are overwritten by sampling (now preparing).