SSIM eval callback for continuous eval

Summary:
Eval Callback design #3 from https://docs.google.com/document/d/1-6HUrsO6pcXW1HDhqowdz_DWziXeGacDekg1BublY_c/edit#heading=h.wrjehs3xh1h4

This diff implements utils and SSIM eval.

Supersedes D50397800

Differential Revision: D50823825

fbshipit-source-id: d9401b24cb3013e4de5edfa70fe78309ef261596

torchtnt/framework/__init__.py

@@ -9,7 +9,7 @@
 from .evaluate import evaluate
 from .fit import fit
 from .predict import predict
-from .state import PhaseState, State
+from .state import ActivePhase, EntryPoint, PhaseState, State
 from .train import train
 from .unit import EvalUnit, PredictUnit, TEvalUnit, TPredictUnit, TrainUnit, TTrainUnit
 
@@ -20,6 +20,8 @@
     "evaluate",
     "fit",
     "predict",
+    "ActivePhase",
+    "EntryPoint",
     "PhaseState",
     "State",
     "train",