Multiple executors when only one task in the session

[k82cn]

Describe the bug

When there're only one running tasks, there're several bound executors.

To Reproduce

Sleep in on_session_enter

Expected behavior

Only one executor

Environment (please complete the following information):

• OS: [e.g. uname -a && cat /etc/os-release ]
• Version: v0.6.0

Additional context

n/a