Type-appropriate null representation

Always use type-appropriate mechanisms to represent the absence of a value. For objects, use default constructors rather than nullptr. For potentially absent values, consider using optional types. For pointers, use null checks before performing operations on them.

Examples:

• INCORRECT: bias_md = nullptr; when bias_md is an object type (dnnl::memory::desc)

• CORRECT: bias_md = dnnl::memory::desc(); to represent an “empty” descriptor

• INCORRECT: ncclHeartbeatMonitorThread_ = std::thread(...); without checking current state
• CORRECT:

  // Check nullness before assignment to prevent issues on second call
  if (!ncclHeartbeatMonitorThread_.joinable()) {
  ncclHeartbeatMonitorThread_ = std::thread(...);
  }
  

This practice improves type safety, prevents null pointer dereferences, and creates more maintainable code by using language features designed for representing absence of values.