Specialize PADDLE_ENFORCE and throw_on_error for pointer types

[wangkuiyi]

By adding the specialization of throw_on_error for pointer types, we can write

PADDLE_ENFORCE(GlobalScope()->FindVar("variable_name"), 
    "variable_name not defined.");

without wrting the checking of nullptr

PADDLE_ENFORCE(GlobalScope()->FindVar("variable_name") != nullptr, 
    "variable_name not defined.");

[gangliao]

Add this template specialization, developers might misuse PADDLE_ENFORCE. For instance, if there is a function test and its return type is not void*, then PADDLE_ENFORCE(test(a, b)) might cause non-deterministic problem. @wangkuiyi

[wangkuiyi]

Sorry but why is this case non-deterministic? Which throw_on_error specialization would this case match?

[gangliao]

@wangkuiyi

For this case, it's definitely ok. We can support this.

PADDLE_ENFORCE(GlobalScope()->FindVar("variable_name"), 
    "variable_name not defined.");

I mean it might give people some kind of feeling that every function in Paddle could use PADDLE_ENFORCE directly.

int test(int a, int b) {...};
PADDLE_ENFORCE(test(1,  2)); // return value > 0, success or fail ?
PADDLE_ENFORCE(test(2,  1)); // return value < 0, success or fail ?
PADDLE_ENFORCE(test(2,  2)); // return value == 0, success or fail ?

But it seldom happens, we can ignore this.

[gangliao]

@wangkuiyi

Also, I want to make it clear that where we need to use PADDLE_ENFORCE and PADDLE_ASSERT.

check the legality of input and output: PADDLE_ASSERT
check the correctness of function execution: PADDLE_ENFORCE

or only use PADDLE_ENFORCE to make it catchable in python.

[wangkuiyi]

Got it. Yes, let's remove the added specialization in PR #3116.