Factory Class for common positional encoding methods

[CCranney]

Recent changes have made adding customized numeric or positional embedding methods easy via the numeric embedding manager class. This was done with the strategy design pattern. That being said, it may still be tedious for end users to create implementations of the desired method each time. I propose making a factory class that would take a simple string and create the desired class. That way the end user just need to indicate the type of positional encoding, and the factory will do the rest.