Chang API of ti.append and ti.length

[neozhaoliang]

Feature 1

The current usage of ti.append and ti.length are quite confusing. They require the users explicitly query the parent node:

import taichi as ti
ti.init()
x = ti.field(int)
block = ti.root.dense(ti.i, 10)
pixel = block.dynamic(ti.j, 10)
pixel.place(x)
@ti.kernel
def test():
    for i in range(10):
        for j in range(i):
            ti.append(x.parent(), i, j)
test()

Shall we change this behaviour to x[i].append(j)?

Feature 2

Support appending struct to dynamic snodes. (currently only 32-bit data is allowed)

Feature 3: auto detect axes

When defining a dynamic snode, one need to explicitly write down the axes:

ti.root.dense(ti.ij, 10).dynamic(ti.k, 10)

Shall we automatically detect the first available axes (with least index) and pend the dynamic snode to this axes?
For example in the above code, ti.k is the first axes below ti.ij, hence we can make ti.k to an optional argument here?

Feature 4: dynamically allocating a dynamic snode

something like this:

@ti.kernel
def test():
    q2 = ti.root.dynamic(10)
    print(len(q))  # Use the __len__ in Python
test()

Feature 5:

Add pop method so that it can return the last active item and then deactivate it.

Feature 6: slicing

support index slicing so that q[start: end] returns a 1D vector field.

[yuanming-hu]

Features 4 and 6 would be a little hard to implement (considering we need to implement these on GPUs).

In the long run, we should consider feature 4, allowing users to allocate fields in kernels.