…rArg`.

`BufferArg` is used to describe parameters passed to the codegen: it
indicates whether the parameter is a var or a buf and holds a pointer to
the corresponding var/buf. Both var and buf contain dtype, and thus
duplicating it in BufferArg is unnecessary - we can always get it from
the var/buf. Hence we're removing dtype_ field from BufferArg in this
PR. We're also adding a `buf_` field here: this is done so that
BufferArg truly has all the info about the parameter.

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…eGen::BufferArg`."

`BufferArg` is used to describe parameters passed to the codegen: it
indicates whether the parameter is a var or a buf and holds a pointer to
the corresponding var/buf. Both var and buf contain dtype, and thus
duplicating it in BufferArg is unnecessary - we can always get it from
the var/buf. Hence we're removing dtype_ field from BufferArg in this
PR. We're also adding a `buf_` field here: this is done so that
BufferArg truly has all the info about the parameter.

Differential Revision: [D28128329](https://our.internmc.facebook.com/intern/diff/D28128329)

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