Is the pilot required to keep visual lookout during IFR operations in VMC? During VFR operations, the pilot must always self-assure separation from other aircraft, and thus must maintain a visual lookout all the time.

For IFR operations but in VMC, are there any rules as to pilot visual lookout? Is the pilot still required to maintain visual lookout (might be difficult in clouds, though...). Even if ATC assures separation, some objects such as drones may not be known to ATC, so ATC cannot provide separation...
You didn't mention which country or jurisdiction you're asking about, but in the US 14 CFR 91.113(b) says:


  When weather conditions permit, regardless of whether an operation is
  conducted under instrument flight rules or visual flight rules,
  vigilance shall be maintained by each person operating an aircraft so
  as to see and avoid other aircraft


I assume other countries have similar regulations but even if they don't, it's still common sense and good piloting to keep a visual lookout whenever possible.