Physics::Elasticity::Kinematics::w: change intermediate value to 'auto'

Physics::Elasticity::Kinematics::w: change intermediate value to 'auto'

This fixes a bug uncovered by our recent base::Tensor() cleanup: The
intermediate tensor type used for the gradient in the w() function
erroneously casts the result to a Tensor<1, dim, double> which for
example for automatic differentiation then removes the underyling AD
type:

/srv/temp/testsuite-d8F1wK0J/dealii/include/deal.II/base/tensor.h:1316:13: error: cannot convert 'const value_type' (aka 'const Sacado::Fad::Exp::GeneralFad<Sacado::Fad::Exp::DynamicStorage<double>>') to 'value_type' (aka 'double') without a conversion operator
  : values{{value_type(initializer[indices])...}}
            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Passes all kinematics/physics_functions tests in the testsuite

Fixes #16517