Fix various bugs in ckpt to diffusers conversion script

[lstein]

What type of PR is this? (check all applicable)

• [X ] Bug Fix

Have you discussed this change with the InvokeAI team?

• [ X] Yes
• No, because:

Have you updated all relevant documentation?

• Yes
• [ X] No

Description

This PR fixes several issues with the 3.0.0 conversion script:

• Handles checkpoint variants that don't put dots between fields in the long state dict key names
• Handles ema, non-ema, pruned and non-pruned ckpts
• Does not add safety checker to converted checkpoints
• Respects precision of original checkpoint file