v0.6.4 🐞 Bug fix

Fix a bug that cannot set seq_len==n_steps for seq_missing().

What's Changed

• Avoid empty step_idx list when n_steps==seq_len by @WenjieDu in #46
• Release v0.6.4 by @WenjieDu in #47

Full Changelog: v0.6.3...v0.6.4