Invoke materialize() only before calling a Taichi kernel

Concisely describe the proposed feature

Originally, materialize() was meant to be invoked immediately before the invocation of a @ti.kernel:

https://github.com/taichi-dev/taichi/blob/9c108ff0e1ce51ba991f1bd4476892d3823fe13a/python/taichi/lang/impl.py#L294-L317

That is, this part is where materialize() is supposed to be called: https://github.com/taichi-dev/taichi/blob/711c299ca9abce4b8a632922ff89295c6939faf3/python/taichi/lang/kernel_impl.py#L383

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It is now scattered in several places in the codebase, see

https://github.com/taichi-dev/taichi/search?l=Python&q=%22materialize%28%22

Describe the solution you'd like (if any)

The reasons are two-fold

1. We used to have to call materialize() before collecting SNode's information. shape has been fixed by @strongoier 's work (#2600) now. However, there are still some places left. We will be happy to guide for the rest of the fix.

• https://github.com/taichi-dev/taichi/blob/0fa4c675a00fcb8eb2993dac427e7818aa7b04fe/python/taichi/lang/snode.py#L295-L305
• https://github.com/taichi-dev/taichi/blob/0fa4c675a00fcb8eb2993dac427e7818aa7b04fe/python/taichi/lang/field.py#L201-L205

2. There are places where an exception should be thrown if the program is not materialized yet (instead of doing an implicit materialization):

• https://github.com/taichi-dev/taichi/blob/711c299ca9abce4b8a632922ff89295c6939faf3/python/taichi/lang/__init__.py
• https://github.com/taichi-dev/taichi/blob/efd66b57fd4438e3fad88450a55b2286e1e249c1/python/taichi/aot/module.py#L79

Additional comments Add any other context or screenshots about the feature request here.