Var1 = tf.Variable(...)
Var2 = tf.Variable(...)
Y_hat = tf.matmul(Var1, Var2)
loss = tf.reduce_sum(0.5*(target-Y_hat)**2)

# Will try to optimize for Var1 and Var2
optimizer = tf.train.AdamOptimizer(0.001)

# For training on basis of Var1
optimize_var1 = optimizer.minimize(loss, var_list=[Var1])