  We present spectro-polarimetric analysis of \thisgrb\ using data from \asat,
\fermi, and \swift, to provide insights into the physical mechanisms of the
prompt radiation and the jet geometry.
  Prompt emission from \thisgrb\ was very bright (fluence
$>10^{-4}$~ergs~cm$^{-2}$) and had a complex structure composed of the
superimposition of several pulses. The energy spectra deviate from the typical
Band function to show a low energy peak $\sim 15$~keV --- which we interpret as
a power-law with two breaks, with a synchrotron origin. Alternately, the prompt
spectra can also be interpreted as Comptonized emission, or a blackbody
combined with a Band function. Time-resolved analysis confirms the presence of
the low energy component, while the peak energy is found to be confined in the
range of 100--200~keV.
  Afterglow emission detected by \fermi-LAT is typical of an external shock
model, and we constrain the initial Lorentz factor using the peak time of the
emission. \swift-XRT measurements of the afterglow show an indication for a jet
break, allowing us to constrain the jet opening angle to $>$ 6$\degr$.
  Detection of a large number of Compton scattered events by \asat-CZTI
provides an opportunity to study hard X-ray polarization of the prompt
emission. We find that the burst has high, time-variable polarization, with the
emission {\bf have higher polarization} at energies above the peak energy.
  We discuss all observations in the context of GRB models and polarization
arising due to {\bf due to physical or geometric effects:} synchrotron emission
from multiple shocks with ordered or random magnetic fields, Poynting flux
dominated jet undergoing abrupt magnetic dissipation, sub-photospheric
dissipation, a jet consisting of fragmented fireballs, and the Comptonization
model.
