[refactor] Remove PyTaichi.compiled_functions (taichi-dev#7867)

Issue: taichi-dev#7002 

### Brief Summary

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copilot:summary
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### <samp>🤖 Generated by Copilot at 5923f65</samp>

Refactor compiled function management for kernels in `PyTaichi`. Use
`compiled_kernels` attribute of each kernel instead of
`compiled_functions` attribute of `PyTaichi`.

### Walkthrough

<!--
copilot:walkthrough
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### <samp>🤖 Generated by Copilot at 5923f65</samp>

* Remove the `compiled_functions` attribute from the `PyTaichi` class
and use the `compiled_kernels` attribute of each kernel instead
([link](https://github.com/taichi-dev/taichi/pull/7867/files?diff=unified&w=0#diff-99744c5ae5f6a754d6f68408fdc64fb0d6097216518a7f3d1ef43ffe12599577L316))
* Update the `clear_compiled_functions` method of the `PyTaichi` class
to clear the `compiled_kernels` attribute of each kernel
([link](https://github.com/taichi-dev/taichi/pull/7867/files?diff=unified&w=0#diff-99744c5ae5f6a754d6f68408fdc64fb0d6097216518a7f3d1ef43ffe12599577L339-R339))
* Update the `get_num_compiled_functions` method of the `PyTaichi` class
to sum up the lengths of the `compiled_kernels` attribute of each kernel
([link](https://github.com/taichi-dev/taichi/pull/7867/files?diff=unified&w=0#diff-99744c5ae5f6a754d6f68408fdc64fb0d6097216518a7f3d1ef43ffe12599577L354-R357))

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

python/taichi/lang/impl.py

@@ -313,7 +313,6 @@ class PyTaichi:
     def __init__(self, kernels=None):
         self.materialized = False
         self.prog = None
-        self.compiled_functions = {}
         self.src_info_stack = []
         self.inside_kernel = False
         self.compiling_callable = None  # pointer to instance of lang::Kernel/Function
@@ -336,7 +335,8 @@ def initialize_fields_builder(self, builder):
         self.unfinalized_fields_builder[builder] = get_traceback(2)
 
     def clear_compiled_functions(self):
-        self.compiled_functions.clear()
+        for k in self.kernels:
+            k.compiled_kernels.clear()
 
     def finalize_fields_builder(self, builder):
         self.unfinalized_fields_builder.pop(builder)
@@ -351,7 +351,10 @@ def validate_fields_builder(self):
             )
 
     def get_num_compiled_functions(self):
-        return len(self.compiled_functions)
+        count = 0
+        for k in self.kernels:
+            count += len(k.compiled_kernels)
+        return count
 
     def src_info_guard(self, info):
         return SrcInfoGuard(self.src_info_stack, info)