add time stamp in load & save function, in file src\components\agents\agent.jl

[norci]

I'd like to save multiple agents during training, in order to pick the best one.

but the save function overwrites the previous one.
I think this can be solved by adding a suffix in the file name, such as use a time stamp.

ref:
keras's ModelCheckpoint
Flux's Checkpointing

[findmyway]

Load/Save related functions are removed from RLCore to keep things simple and concise.

JuliaReinforcementLearning/ReinforcementLearningCore.jl#169

but the save function overwrites the previous one.
I think this can be solved by adding a suffix in the file name, such as use a time stamp.

One can manually implement it as a customized hook.

[norci]

agree.
we can save the weights of the model only, and load it into the policy.