How to prove that the set of matrices of rank not greater than r is a smooth manifold?

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How to prove that the set of matrices of rank not greater than $r$ is a smooth manifold? That is to prove that $S=\left\{\boldsymbol{X}\in\mathbb{R}^{n_{1}\times n_{2}}: \mathrm{rank}(\boldsymbol{X})\leq r\right\}$ is a smooth manifold. What is its dimension?