don't silently drop symbols without implementation

[t-vi]

Currently, if a symbol does not have an implementation, it is silently dropped in transform_for_execution.

This was one factor of making it hard to debug #1166 and is quite a UX hazard when things go wrong for whatever reason.

cc @apaz-cli

[IvanYashchuk]

What is meant by "does not have an implementation"? No meta function

lightning-thunder/thunder/core/symbol.py

Line 130 in a15d586

meta: Callable | None = None

or no python_impl function

lightning-thunder/thunder/core/symbol.py

Line 139 in a15d586

python_impl: None | Callable = None

?

Can you provide a minimal example of problematic behavior and what change would you like to see?