Power In= Power Out in transformers
But when it comes to currents, we show that \$\dfrac{Ip}{Np} = -\dfrac{Is}{Ns}\$


Yes, that relationship is true: -



But it doesn't mean that the power out to the load is negative (see the above picture). It just means that as far as the transformer is concerned, the current is flowing outward with respect to the primary winding.

The 1 ohm load still receives positive power as if it were fed from a voltage source. The secondary produces a current opposite to the primary (in that the current is flowing away from the transformer) but, it is still flowing in the correct direction for positive power into the resistor load.


  Beside this we say that Power supplied to the primary coil is equal to
  the power supplied to the second coil.


An ideal transformer doesn't deal with watts; it deals with volts and amps so we talk about \$VA\$ and that makes it clearer in my book. Power is supplied to the load; it isn't supplied to the secondary and it isn't really supplied to the primary. Both primary and secondary deal in volts and amps. That's why we talk about transformers having a \$VA\$ rating.