  The Initial Mass Function (IMF) for massive galaxies can be constrained by
combining stellar dynamics with strong gravitational lensing. However, this
method is limited by degeneracies between the density profile of dark matter
and the stellar mass-to-light ratio. In this work we reduce this degeneracy by
combining weak lensing together with strong lensing and stellar kinematics. Our
analysis is based on two galaxy samples: 45 strong lenses from the SLACS survey
and 1,700 massive quiescent galaxies from the SDSS main spectroscopic sample
with weak lensing measurements from the Hyper Suprime-Cam survey. We use a
Bayesian hierarchical approach to jointly model all three observables. We fit
the data with models of varying complexity and show that a model with a radial
gradient in the stellar mass-to-light ratio is required to simultaneously
describe both galaxy samples. This result is driven by a subset of strong
lenses with very steep total density profile, that cannot be fitted by models
with no gradient. Our measurements are unable to determine whether $M_*/L$
gradients are due to variations in stellar population parameters at fixed IMF,
or to gradients in the IMF itself. The inclusion of $M_*/L$ gradients decreases
dramatically the inferred IMF normalisation, compared to previous lensing-based
studies, with the exact value depending on the assumed dark matter profile. The
main effect of strong lensing selection is to shift the stellar mass
distribution towards the high mass end, while the halo mass and stellar IMF
distribution at fixed stellar mass are not significantly affected.
