Add a default rule for custom blocks

[radhakrishnatg]

Fixes # .

Summary/Motivation:

Add a default rule for custom blocks. With this default rule, construction of custom blocks becomes easier.

@declare_custom_block(FooBlock)
class FooBlockData(BlockData):
    def build(self, *args, option_1, option_2):
        self.x = Var()
        self.y = Param(initialize=option_1)
        
        
m.blk = FooBlock([1, 2, 3], options={"option_1": 1, "option_2": 2})

Implementing build method is optional. If it is not implemented, an empty block will be returned.
Users can overwrite the default build method by passing the rule argument:
m.blk = FooBlock([1, 2, 3], rule=my_custom_block_rule) # Ignores the build method

Changes proposed in this PR:

• Added default rule argument to the declare_custom_block decorator
• Added tests to cover the changes.

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