JIT errors do not change Variable values

[chaoming0625]

Previously, errors occured in JIT function will change value contents of all variables.

>>> import brainpy.math as bm
>>>
>>> a = bm.Variable(bm.zeros(1))
>>> a = bm.zeros(1)
>>> @bm.jit
>>> def f():
>>>    a.value += 1
>>>    b.value += 1
>>> f()
brainpy.errors.MathError: JaxArray created outside of the jit function cannot be updated in JIT mode. You should mark it as brainpy.math.Variable instead.
>>> a
Variable(Traced<ShapedArray(float32[1])>with<DynamicJaxprTrace(level=0/1)>)

Now, the content will keep the same. For example,

>>> import brainpy.math as bm
>>>
>>> a = bm.Variable(bm.zeros(1))
>>> a = bm.zeros(1)
>>> @bm.jit
>>> def f():
>>>    a.value += 1
>>>    b.value += 1
>>> f()
brainpy.errors.MathError: JaxArray created outside of the jit function cannot be updated in JIT mode. You should mark it as brainpy.math.Variable instead.
>>> a
Variable([0.], dtype=float32)