Improve error message when function local variables are GC'd.

New message:

```
ValueError: A tf.Variable created inside your tf.function has been garbage-collected. Your code needs to keep Python references to variables created inside `tf.function`s.

A common way to raise this error is to create and return a variable only referenced inside your function:

@tf.function
def f():
  v = tf.Variable(1.0)
  return v

v = f()  # Crashes with this error message!

The reason this crashes is that @tf.function annotated function returns a **`tf.Tensor`** with the **value** of the variable when the function is called rather than the variable instance itself. As such there is no code holding a reference to the `v` created inside the function and Python garbage collects it.

The simplest way to fix this issue is to create variables outside the function and capture them:

v = tf.Variable(1.0)

@tf.function
def f():
  return v

f()  # <tf.Tensor: ... numpy=1.>
v.assign_add(1.)
f()  # <tf.Tensor: ... numpy=2.>
```

PiperOrigin-RevId: 225957777