[Unity][nnModule] Dynamic shape support in nn Module

[CharlieFRuan]

This PR allows dynamic shape in some modules of nn.Module (Linear and Embedding), motivated by dynamic vocab size as discussed in mlc-ai/mlc-llm#1417.

UX-wise
We allow nn.Parameter("vocab_size", 4096), equivalent to nn.Parameter(tir.Var("vocab_size", "int64"), 4096).

Symbolic relationship across Parameters
We make sure the same symbolic shape for different nn.Parameter do not appear twice within the same method. For instance, the following would not happen:

@R.function
def prefill(...):
  vocab_size = T.int64()   # for `embed_tokens`
  vocab_size1 = T.int64()  # for `lm_head`

This is prevented by str2var_params in Exporter, so that all embed_tokens.shape[0] and lm_head.shape[0] share the same tir.Var.

Symbolic vars shared across functions
We also prevent two relax.Function sharing the same tir.Var for a symbolic shape, e.g. TIR variable A and B share the same physical address in memory below.

@I.ir_module
class Module:

  @R.function
  def prefill(...):
    vocab_size = T.int64() <== A
    ...

  @R.function
  def decode(...):
    vocab_size = T.int64() <== B
    ...

We prevent this by reinitializing params = _params() and the mapping before each call to _emit_method(), so that the packed_params is different for each function.