When do we use $\boldsymbol\tau=I \boldsymbol\alpha$ and $\boldsymbol\tau=\mathbf r \times \mathbf F $? Is the answer to both comes the same? Is the torque of a body about an axis and the torque due to a force at a distance the same thing? Is the torque at a point by a force equal to the torque of the whole body? 
 A: The equation
$$\boldsymbol\tau=\mathbf r\times\mathbf F$$
Is the definition of the torque $\boldsymbol\tau$ of a force $\mathbf F$ that is applied at position $\mathbf r$ relative to some "reference point" (where $\mathbf r=0$).
The equation
$$\boldsymbol\tau=I\boldsymbol\alpha$$
Is an analog of Newton's second law ($\mathbf F=m\mathbf a$). Therefore, in this equation $\boldsymbol\tau$ represents the net torque $\sum_i\boldsymbol\tau_i$.

Is the torque at a point by a force is equal to torque of the whole body?

This question is not very clear. But I think I can still help based on what we have defined above. If you have a body with a set of forces $\mathbf F_i$ acting on it, you can determine the torque $\boldsymbol\tau_i=\mathbf r_i\times\mathbf F_i$ about some point$^*$ for each of the forces. Then you can find the net torque acting on the body by adding up all of these torques. $\sum_i\boldsymbol\tau_i=I\boldsymbol\alpha$

$^*$ You can choose any point you want, but some points are better than others depending on the analysis you want to do on your system.
