[bug] Fix MatrixFreeCG so it can handle multiple input sizes.

[houkensjtu]

Brief Summary

Previously, MatrixFreeCG inappropriately assume that the intermediate vectors used in CG solver are all two-dimensional ti.field. This is incorrect because the dimension of Ap p and r should be consistent with input b and x.

Walkthrough

Instead of simply put vector_fields_builder.dense(ti.ij, size).place(p, r, Ap), now it's based on the length of size.

def MatrixFreeCG(A, b, x, tol=1e-6, maxiter=5000, quiet=True):
    ...                                                                                                                                                                          
    vector_fields_builder = ti.FieldsBuilder()                                                                                                                                                                                          
    p = ti.field(dtype=solver_dtype)                                                                                                                                                                                                    
    r = ti.field(dtype=solver_dtype)                                                                                                                                                                                                    
    Ap = ti.field(dtype=solver_dtype)                                                                                                                                                                                                   
    if len(size) == 1:     # Determine the `axes` argument based on the length of `size`                                                                                                                                                                                                             
        axes = ti.i                                                                                                                                                                                                                     
    elif len(size) == 2:                                                                                                                                                                                                                
        axes = ti.ij                                                                                                                                                                                                                    
    elif len(size) == 3:                                                                                                                                                                                                                
        axes = ti.ijk                                                                                                                                                                                                                   
    else:                                                                                                                                                                                                                               
        raise TaichiRuntimeError(f"MatrixFreeCG currently cannot support {len(size)}-D inputs.")                                                                                                                                        
    vector_fields_builder.dense(axes, size).place(p, r, Ap) 

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[neozhaoliang]

LGTM!