Improve handling of reported values.

[sile]

This PR includes the following changes for Trial.report method:

• Convert reported values to float type by using float function:
  • Although the type hint of value argument is just float, this conversion is probably desirable for some users (Doesnt work with MySql or PostgreSql #642, Pruning + Distributed optimization #655), and
  • This behavior is consistent with the handling of objective values in Study (study.py#L549)
• Raise an error if the above conversion cannot be applied:
  • This change will make it easier for users to identify the cause of problems with malformed reporting values (suggest_discrete_uniform error "unsupported operand type(s) for *: 'int' and 'NoneType'" #471, Prevent TensorFlowPruningHook from reporting None. #481)

[g-votte]

LGTM

[hvy]

LGTM!