$\DeclareMathOperator\At{At}\DeclareMathOperator\Obj{Obj}\DeclareMathOperator\Mor{Mor}$According to https://ncatlab.org/nlab/show/Atiyah+Lie+groupoid#idea the *Atiyah Lie groupoid* $\At(P)$ of a principal $G$ bundle $\pi:P \rightarrow X$ is a category for which $$\Obj(\At(P))=\lbrace \pi^{-1}(x): x \in X \rbrace$$ and $$\Mor(\At(P))=\big\lbrace f:\pi^{-1}(x)\rightarrow \pi^{-1}(y): \text{$f$ is a $G$ equivariant morphism}\big\rbrace.$$ Structure maps of this category are easy to guess. Now it is easy to see that $\At(P)$ is indeed a groupoid. 

Although it is mentioned in https://ncatlab.org/nlab/show/Atiyah+Lie+groupoid#idea that the *Atiyah Lie groupoid* is indeed a Lie groupoid, I am not able to guess  appropriate smooth structures on $\Obj(\At(P))$ and $\Mor(\At(P))$ such that the source and the target maps are surjective submersions and other structure maps are smooth. 

Is there any **natural choice** of such smooth structures on both $\Obj(\At(P))$ and $\Mor(\At(P))$ such that $At(P)$ is a Lie groupoid so that if someone talks about the *Atiyah Lie groupoid* of a principal $G$ bundle then he/she is precisely assuming **those natural choice of smooth  structures** on $\Obj(\At(P))$ and $\Mor(\At(P))$?

I would  also be very grateful if someone point me to any literature in this direction.