Generic cleanup Metrics and Losses

[JimClarke5]

This PR Is a reissue of the Generic Cleanup PR #203, removing commits in the branch that are already in master.

This PR effects both losses and metrics.

This PR cleans up generic parameters to cut down on the number of unique parameters to at most 1 where possible.
For example:
public abstract class Metric<U extends TNumber, T extends TNumber> {
becomes
public abstract class Metric<T extends TNumber> {

and

@Override
  public <V extends TNumber> List<Op> updateStateList(Operand<U> values, Operand<V> sampleWeights) {

Changes to:

 public List<Op> updateStateList(
      Operand<? extends TNumber> values, Operand<? extends TNumber> sampleWeights) 

[karllessard]

Hi @JimClarke5 , there are some javadoc issues in this PR, please take a look: https://github.com/tensorflow/java/pull/204/checks?check_run_id=1825661890

[karllessard]

BTW, do you have any clue why the javadoc has been reformatted? It looks like it was already conform before, I don't know why your IDE plugin would reformat these, can you please just double-check that you use your original settings?

[JimClarke5]

@karllessard I fixed all the JavaDoc issues under framework except for the data package.
I have no clue why the format was messed up, but I applied IntelliJ Google format on the optimizers, losses, metrics and utils directories.

[karllessard]

Great work @JimClarke5 , that simplifies really a lot some of the APIs. I just left a really-unimportant comment about something that looks like a trailing character, please confirm and/or fix and we are good to merge

[karllessard]

Ok nevermind, I've remove the trailing character directly from your branch, this is now ready to merge, thanks!