Replace instances of `control_dependencies(None)` with `init_scope` when

initializing variables.

Today, when variables are constructed (Resource and otherwise), we lift certain
operations, including the VarHandleOp and initialization ops, out of all
control flow contexts; the mechanism for doing so is entering the context
manager returned by `control_dependencies(None)`. This change replaces various
instances of this mechanism with `init_scope`, which clears control dependencies, lifts ops out of function-building graphs, and pauses the gradient tape. As a result,
variables that are created inside graph functions will be automatically hoisted
into an outer context.

PiperOrigin-RevId: 183320576