refactor: improve Checkpoint-Serialize consistency for state_dict

[rwtarpit]

Fixes #{issue number}
NA
Description:
Refactored Checkpoint.state_dict and Checkpoint.load_state_dict to remove hard-coded "_saved" strings.
Uses base class Serializable's validation mechanism to verify state_dict.
Check list:

• New tests are added (if a new feature is added)
• New doc strings: description and/or example code are in RST format
• Documentation is updated (if required)

[vfdev-5]

Thanks @rwtarpit !