It won't. Not to say that nuking the planet won't end the vast majority of life, but you can't ignite the atmosphere. There is no other substance in the air the oxygen can react with, the nitrogen is inert and to create the conditions for nuclear fusion, you need so much power in the first place, that it doesn't make a difference in the end. Even if you manage to locally create the condition for fusion (as in a hydrogenbomb), the energy will just dissipate and scatter too fast to sustain a chainreaction. Earth is just too big and too cold for that.