Speed up task registry by using lazy properties

[copybara-service]

Speed up task registry by using lazy properties

Functionality:
No change.
The preprocessors and metric_fn attributes of Task are now lazy. This is where many of the slow inspect calls were being invoked.

Speed:
Before making changes: ~120s for importing large bundle of tasks
After making changes: ~60s for importing large bundle of tasks
Unsurprisingly, different usages of these APIs result in differing performance, but no performance degradation is currently observed.