[Hybrid Script] Misc fixation

[were]

3 major things:

1. Add a symbol table check to avoid id name conflict.
2. https://discuss.tvm.ai/t/bug-storage-rewrite-doesnt-correctly-handle-local-buffer-allocation/1664/2
   https://discuss.tvm.ai/t/hybrid-variables-on-cuda-should-have-local-scope/1510
   Follow up these issues cast by @vinx13, now hybrid script should inject the thread extent attribute right under the first global scope of the variable and as outside as possible to work around the attached scope problem.
3. https://discuss.tvm.ai/t/hybrid-script-gpu-schedule/1720
   Follow up the issue cast by @Laurawly, essentially this issue is because TVM has NO abstract to access CUDA registers. If you are writing CUDA, you can just do what it is in normal C++, defining local variables, but in HalideIR there are no variable variables.
   To avoid confusing further users, now compiler will throw error and hint: defining local variables under a bind body is not allowed, and define an array of scratchpad is encouraged.

1 minor thing:
Rename var_decl.py to preprocessor.py

[tqchen]

Thanks, @were @vinx13 @Laurawly this is merged. @were in the spirit of getting more folks working on the same thing, it will also be great if you can mentor someone to help patch and review hybrid part as well