How tf.gradients work in TensorFlow

Given I have a linear model as the following I would like to get the gradient vector with regards to W and b.

# tf Graph Input
X = tf.placeholder("float")
Y = tf.placeholder("float")
# Set model weights
W = tf.Variable(rng.randn(), name="weight")
b = tf.Variable(rng.randn(), name="bias")
# Construct a linear model
pred = tf.add(tf.mul(X, W), b)
# Mean squared error
cost = tf.reduce_sum(tf.pow(pred-Y, 2))/(2*n_samples)

However if I try something like this where cost is a function of cost(x,y,w,b) and I only want to gradients with respect to w and b:

grads = tf.gradients(cost, tf.all_variable())

My placeholders will also be included (X and Y). Even if I do get a gradient with [x,y,w,b] how do I know which element in the gradient that belong to each parameter since it is just a list without names to which parameter the derivative has be taken with regards to?

In this question I'm using parts of this code and I build on this question.