Anharmonic lattice dynamics in large thermodynamic ensembles with machine-learning force fields: <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>Cs</mml:mi><mml:mi>Pb</mml:mi><mml:msub><mml:mi>Br</mml:mi><mml:mn>3</mml:mn></mml:msub></mml:mrow></mml:math>, a phonon liquid with Cs rattlers